Uhh, the first one is questionable. It has some good elements but some serious flaws. The sequel looks to be significantly better in every way, and it will also include the entire first game on the disc (with all the gameplay improvements of the sequel).
Honestly, just from the gameplay improvements they've made in the sequel, I could confidently say that WKC2 will be an absolutely amazing game for online multiplayer (which is the meat of the game).
I wouldn't recommend playing WKC1 now unless you just want to get your feet wet, and I certainly wouldn't advise spending any money to do that. Either wait for the NA release which is slated for August, or import the EU release sometime in early June. Since it's very focused on multiplayer, it's one of those games that's much better to get into near release. That said, there was still a decent online community in WKC1 a year after its release, and you don't even need that many people to be able to fully experience WKC.
I'll also note that the singleplayer campaign is pretty unremarkable. It's really just a springboard to get you ready for online play. The singleplayer campaign is something like 20-30 hours of play. Finishing everything on online could easily take you 300-500 hours depending on your efficiency (it's a huge grind, but a huge fun multiplayer grind).
It scratches a similar itch that PSO does, except that instead of hunting for red boxes you're hunting for crafting materials to upgrade your shit. Though apparently in WKC2 you can find weapon drops as well, so...
Whatever, fuck. Just wait for WKC2 if anything.
Edit: I forgot, there's also a psp game coming out in early june I think, that might also be a decent way of figuring out if you might be interested in WKC2. I haven't played it yet though so I can't really attest to that. It might be awful.