I didn't get either game, but I borrowed and rented them before. I beat both and loved them a lot.
Colloseum is my favorite of the two and I still have my Umbreon from my very first playthrough! I really loved the double battles that pretty much made up the game, which really made battles more challenging and fun. I actually had fun going to Mt. Battle (or whatever) and defeating trainers one by one, since it gave experience. I also loved some of the music tracks.
If they ever make a game like it for the Wii or Wii U, I'll definitely be on board.