Really simple. What would you do with 20 euros of PSN money. Would you get the expansion of inFamous, would you use your creativity in Minecraft or get the first part of the Metro series. 

A week ago I wanted to get inFamous, yesterday on the brink of getting Minecraft but maybe trying out the Metro series could be great as well. What is the best value and how is Metro holding up against non remake games? I have infamous Second Son and I really liked it. I played Minecraft on the PC a couple of times but it's not the kind of game that I can play on my desk, but I really like exploring. And I like thrilller, horror, and FPS so maybe Metro is the great choice. 

If there are other games that I should get instead, please write in comments. 

This game that I'm going to buy will be the game (next to my current games) I'll play until DriveClub releases. Destiny will be for another time since I'll wait out until it gets cheaper. 

All are probably good choices. The inFAMOUS expansion is likely the shortest of those 3 though. Minecraft can definitely be played the most. Metro 2033 is quality.

Get Oddworld: New N Tasty if you haven't already. I can't really choose out of the one's you've suggested as i haven't played any of them yet. But i'm gonna pick up First Light when it releases at retail which i think is the September 10th.

I support the notion above me of buying Oddworld New N Tasty, but it's a bit more expensive than 20€ I think.

From the list, you probably know what to expect from Minecraft, if you like it it's great value as people play it for years.
Metro 2033 wasn't as good as Last Light imo, it felt to me that the budget was constrained and mostly went towards graphics, didn't finish it yet, maybe will give it another chance later. Actually hope it comes to PS+ sometime as I would prefer playing it on PS4 over my PC.
First Light is great, really enjoyed it, possibly more than inFamous 2, probably because it was relatively short so I didn't get bored of it, but it had very strong points on it's own. Would highly recommend this one!

If you already have Infamous, wait on the DLC if you want something else. The DLC is good but it feels similar to ISS, adds some arena stuff though.
Get Metro, since Minecraft is best on PC anyway, imo. Minecraft will also be the first to drop in price I believe.

I would save it up for a sale... but if i absolutely had to choose it would probably be Strider first and First Light second.

As for Minecraft I would think the opposite will happen. Was Minecraft ever discounted on 360? It wasn't on PS3.

There's 2 sales ongoing atm, links to a thread I made in my sig, definately go there and check out some of what's on offer (the sale, I left the thread pretty much as links to the sale) but yeah asking this is pretty much like saying what you should spend 20euros on, depends on what you want.

Save it for a sale. That's what I like to do anytime there is extra monies in my PSN wallet

How about Transistor?