Get the first game first.
The second game has sharper textures and there's more variety to the levels (it's prettier), but it's mostly just a series of tiny themed "arenas" (garden level, snow level, school level, campsite level, racetrack level, etc.).
The first game has more "epic" levels that are very carefully designed to take advantage of the large landscape. It's all one world, and the levels cover more ground. You get to see how it all VERY carefully connects together. But the levels in the second game are usually too small and short.
It's a great game, don't get me wrong -- rolling up the sumo wrestler never gets old -- but you can tell the developers were slightly less inspired the second time around. Still, I love rolling up all the international landmarks... awesome.
The music for the second game has some really good tracks, but most of it is just white noise, pops, and random yelling. When I got the soundtrack I thought my CD was skipping.