Which do you prefer, and why?

Metroid Prime 2. Better music (for the most part), more exploration, less linear, and better bosses! Though, Prime 3 isn't too far off.

I would simply say they are different, each strong where the other is weak. Prime 3 might edge it out for me just because I really liked the control scheme, and Prime 2 seemed to drag a bit, though that might have been just the difficulty and my impatience at the time.

No idea. Both of them were really good imo.

Prime 2. It has better replay value.

If I included the first, it would win the popular vote by a landslide, and the poll wouldn't be as fun. 

Haha you noticed that, huh? Yeah, i'll admit i'm a little biased :)

While I love both games, I slightly prefer Metroid Prime 3.

There are 2 things about Echoes that irk me. First of all, I don't enjoy exploring its dark world as much as its light world. Second of all, I hate that you have to wait until the end of the game to collect about 7 of the 9 Sky Temple keys. I'd prefer being able to fight the Ing Emperor right after defeating Quadraxis.

The only thing about Corruption that irks me is the fact that the missiles are pretty much useless thanks to the PED suit and its Hypermode.

I enjoyed both pretty evenly but I guess I'll give it to Prime 2 for the multiplayer. I don't know how others feel about it, but me and my friends always had a blast playing it.

Metroid Prime 3. It's prettier, it's funnier and the ost is better. Of course, that's just my opinion.