you should sell the Vita and get the XL
in a couple of years if the Vita is still around buy it for cheap, it will have more interesting games...

If you really must part ways with your Vita, in any type of transactions I never found trading as advantageous as selling items on an online auction or shopping website such as eBay. However, I have no knowledge of how the market works in any other country other than the United States.
I would have suggested to wait until what Sony has to say at Gamescom, but it looks like you've really thought this through; even though, it still seemed a hasty decision due to your wanting a 3DS XL.

Judging from the OP, it seems Nintendo is more suited for your taste anyhow. On that note, then by all means make the exchange.

Well if money didn't matter you would just buy the 3DS XL straight up. Trading in your Vita for it is a bad decision. Here's why:

1. You already have a 3DS. I know the XL is very tempting but unnecessary at the moment. You should try to find a place that will transfer your stuff over and trade your current 3DS instead.

2. You lose access to all Vita games. Though that might not mean much right now for you, later this year, next year, and beyond that, you are going to want to play those games too. You will probably end up buying a Vita again, so it is going to be expensive. Keep both systems, save your wallet for games.

3. Vita's future functionality is too appealing. You said it yourself, the Vita does things that Nintendon't (had too). PS Mobile will be hella awesome and make the Vita vastly more attractive outside of games, or in small mobile games if you like that, then the 3DS will. I'm talking tons of apps that go neck to neck with iOS and Android. Also Vita has a huge backlog of PSP games, minis, and will be getting PS1 games by at least next month (end of summer).

Basically, your impatience will cost you money. If you don't have 200 to spend right now for a new model of a system you already own, you won't have it down the road when you're kicking yourself for not having a Vita.

You will get a 3DS XL eventually or find a way to trade your 3DS up but do you really want to drop the money on a Vita again?

If you're asking, then you somewhat like the system, and yes, It's a stupid decision.

Trade in your Vita, you'll get a lower price, you'll then transfer your games to your new 3DS, and then sell your old 3DS at a lower price. If the Vita does start doing better and you purchase one later, sure you'll have lost money overall.

Now I'm pretty sure that there's a Gamestop employee who won't be such a douchebag and allow you to transfer your current data over to the new system if you sell a 3DS. If they're afraid that you'll run away with both systems, explain to them and ask if you can show them how the transfer works and if they can do it for you once the trade is complete and you purchase the XL. That way they won't have to worry if they're going to get robbed, they'll be taking the time to do it.

Personally, I'd sell the 3DS on either Ebay or Craig's List and mention it's an Ambassador Program version that comes with the 10 GBA and 10 NES games. Although the ones that interest me I own on the GBA, and have the same games on my PSP through emulation. If the Ambassador's Program is so much to you, it must mean your opinion of the 3DS itself is pretty low if you need those 20 games to have a good line up.

Another reason is you currently have two systems and the only thing that'll happen when you get the XL is you'll have a bigger screen, which is nice, and in a year from now, Nintendo will release the XL dual analog version and you'll want one of those as well. You could always wait until the last of Nintendo's revisions and then purchase that one instead of purchasing each and every revision that comes out. Nintendo likes people like you because each new Nintendo portable, they make two, three, four, or more sales. It's a tactic Nintendo clearly pulls with each and everyone of their portables. These aren't minor changes. Most of the time it's an pretty big change which gives meaning to g o out and purchase the new one.

--

So aside from that. What about PSP games. You mentioned Monster Hunter Freedom Unite. I'm sure there's others you're interested in if you purchase digital copies, and despite not as many in comparison to physical releases, there's a lot of great games.

On the PSP, Phantasy Star Portable and Gods Eater are available, both play similar to Monster Hunter. I'm more of a Phantasy Star Online/Portable fan myself, and Phantasy Star Online 2 for the Vita will be free in Japan, for Vita, and it'll more than likely make a western release what with the PC version heading west as well. There's Half Minute Hero, Valkyria Chronicles, Dracula X Chronicles, Prinny 1 and 2(which have cheaper digital codes being sold at Gamestop,) the Atlus sale that's going on right now with a decent amount of good games, and many more. If you need an idea of great Vita games, I'm sure there are plenty of Youtube videos describing as such.

PSX support will be here this summer, August being the end of summer, so not too far to go. Alundra, Grandia, Blood Omen, Crash Bandicoot, Squaresoft titles, etc.

Then finally, not sure if you own a PS3, but the Vita is the ultimate companion to the PS3. Right now I have about 20 PSX games , the Pixel Junk games, and a few movies on my PS3. If I'm not at home, but at a wifi location(which if not at home I sometimes am,) you can use remote play through the internet and play any of those games from wherever you're at. Your PS3 will remotely turn on and you can start the games up. God of War HD Collection and the ICO/SotC HD Collection is going to be playable through remote play soon as well(Infamous 2 is what I'd like to see playable.) But anyways. Remote play is an amazing feature, and while it'd be better if all games were compatible through remote play it's still a great service despite the shortcomings.

1. Trade in your PS-Vita to get the 3DS XL
2. Transfer your stuff from your 3DS to your 3DS XL
3. Trade in your 3DS for a used PS-Vita

Sounds good tbh.

It sounds stupid, What are you getting having a larger 3DS? Nothing but a larger screen, and losing a Vita at the same time. In about 4 months the price will drop on the 3DSXL, and 8 months from now they'll introduce a new 3ds model. Are you going to trade up for that too? Why not wait, kinda like I did with my IPhone, I got the 3g skipped the 3gs, got the 4 skipped the 4G, and will be getting the 5, if only to sell it, cause after the vita i may not need a smart phone anymore. Anyway, back on topic. I did the same thing awhile back, I traded one system for another, I owned a 64 4-5 times, at least 2 game cubes, maybe 3 dreamcast, I've had over 6 ps2's, 2 xbox 360's. And never once was the trade worth it, and i regretted doing so, Now i just keep the console, Had a PS3 and wii since launch, and i wont trade them when next gen comes either. You'll regret it.

Hmm... that actually sounds like a really good idea. 

Do this.

Just wanted to give you all a heads up that the Vita is safe^^ I discussed the matter with the Gamestop employee/manager (it's a really small store) and I will buy the 3DS XL regularly. Then I will take it home with me, do the transfer and then I will "return" it. I'll buy it again and use the trade in option. And two weeks later I will also get NSMB2 for 5€.

Glad to hear it. Everyone's a winner!