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.
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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.







