Which is the better game, and why?

Havent played either one... so i'm gonna say Sonic 2 and Sonic 3 & Knuckles. xD

Might get Lost world eventually though.

Can I pick Sonic Lost World just because it looks prettier. 

Lost Worlds is better. I own both. I was excited for both and was disappointed in both, but I was way more disappointed in Colors. Such an overrated game, like the whole turd that is the Adventure series. Seriously some of the worst games I've ever played.

But yeah, Lost World was the first 3D Sonic game I've played that was mediocre. Well that's not fair. Colors wasn't bad. Just significantly worse than Lost World. Lost World was fun-ish.

I only played Sonic Lost World, but I can say that it doesn't deserve all the criticism. It's a 8/10 for me, but it's too short and too easy (even with all red rings).

There should have been more post Eggman content like the 3D World post Bowser content. The hidden world levels feel like unfinished prototypes.

Both are pretty bland.
Im not that big of a sonic fan though so maybe jumping into these 2 games first were probably not my best option...
Out of the 2 id say Lost World.
I played it recently on 3ds and Wii U and thought it was ok.
id give it a 6.5/10

Only played Colors and I loved it, I did play the demo of Lost World and if the game is more or less like that then Colors blows it away.  

I actually think an HD version of Colors would look better than Lost World.

Colors knew exactly what it was. A 2D Sonic game with a couple of 3D sections. Lost World throws a lot of things at you, hoping that you'll like at least one of them. 2D levels, 3D levels, tube levels, auto-run levels, snowball levels

Even in SD I prefer Colours over Lost World visually.