get prime 1. Prime 2 isn't quite as compelling (still it's probably the 3rd or 4th best cube game)... if you like 1, wait a little while and play through 2, you don't want to play it too soon because they're both very similair and you'll get a "been there done that" feeling. wait till you're in the mood to replay the first one, and THEN play the second one. Also it's better to play them in order because the storylines intersect.
If you give metroid prime some time you will find that it has all the elements from the 2d games... the structure is identical, it has platforming, probably the best 1st person platforming i've ever seen... it has exploration, puzzles, an expansive world with no levels and no load times... It's incredibly immersive, has great music. awesome boss fights, a perfect challenge.. IMO it's nearly a perfect game and it's my favorite game of this generation.
Echoes is very similair... it's a bit more linear feeling, but still very non-linear... it's a little harder, the maps are more confusing, and the dark/light mechanic can get a little obtrusive. also it feels more like a side story, while the first one feels like a typical metroid story, along the lines of metroid and super metroid.. Still it's a really awesome game, and it only suffers in comparison to the first one.