Yeah, I would agree that it does depend on your musical tastes. If you swing a little more toward the pop and r&b side, KR:AI is the way to go. If you're more of a rocker (especially of the 80s/90s variety) then Rock Band or GH:WT would suit you perfectly. And if you're a 12 year old, then you might enjoy Boogie, Sing It, or HSM, haha.
I own every Karaoke Revolution game and both Rock Bands, and I really don't find the singing portions to be that different in structure. After all, both were created by Harmonix. I also have GH:WT, but haven't done much with the singing...but I do know that it has one of the most fun karaoke songs of all time (Bon Jovi's "Livin on a Prayer").
Overall, I'd say definitely look at the track lists and pick the one you like best. In the end, karaoke is karaoke. It may seem like a waste to buy Rock Band just to use as a singing game. But I know people who buy it only to play the guitar part...and to me, that seems like a waste. It's really just about what you're into. I still think Rock Band gives you the most options, but if you've got friends who want to sing Barry White, or Cher, or Stevie Wonder, then yeah...go for KR:AI.