The Witcher vs. Trine 2 Complete Story (in TF2 Keys)

You can do your own research on /r/steamgameswap, steamtrades, tf2outpost, etc.

Both games you are talking about trade ~1.5-2 keys probably. However, if you actually do go through with the trade, make sure that the Trine 2 copy is actually the Complete Story. Old vanilla Trine 2 copies also say that they are "Complete Story" but they don't actually include the Goblin DLC pack. There was some distinguishing mark, but I forgot what it was. You'll have to look that up.

I wouldn't rely on tf2outpost too much, because most people there lowball as much as they can and it gets ridiculous at times, though you can still refer to a few of the trades there.
imo in terms of raw value the trine 2 complete story (make sure it's complete story) is higher than witcher, though if you are looking to resell the trine 2 for tf2 keys that can take a lot of time and effort and on top of that most people will be waiting for steam sales where the game can go for 75% off (=4.99 USD) which is only around 2 keys
but that's just my perspective

Trine 2! I've played both and Trine 2 is a lot funner. Its multiplayer is where its at. I'm not a huge fan of most indie games, but Trine 2 really got me interested. I played the shit out of it and beat it in 3 days haha. Very fun game

Someone was selling Trine 2 Complete Story for 1 key at TF2Op when it was on sale, not sure about The Witcher.

both 1,5-2 keys. These are no games you can make profit with, better play it yourself.