EDIT: ahh no one has rock band yet. Do you think RB is better than guitar hero?
Not sure what the differences would be between the ps3/360 versions and the wii version, but i definitely think rb2 is better than ghwt. rb2 has a much bigger music library, and a more diverse one at that. the new instruments are great, and although this is subjective i believe the overall style/presentation of rb2 is much better than ghwt. i'd also like to note that i loved guitar hero 3 (it was my first fake-instrument game), then got rock band 1 and later rock band 2, and finally bought ghwt and hated it. even Tool couldn't save the game for me, and i used the "all quickplay songs" cheat just to get to their songs . . . on to
some of the reasons i can think of right now, that aren't subjective.
as a band, failing the entire song just because one person isn't that great really blows, but that's how ghwt does it. rock band 2 has a three strikes system; you can save a player that failed out by using your overdrive ("star power"), and you can only save them twice, giving them a total of 3 "plays." once a person completely fails out, you have a certain amount of time left to play before the entire band fails on the basis that . . . you're missing a player, haha. i think this is better because you can get through certain insane parts of songs on a certain difficulty (fail out, wait until insane part is over, then have your friends bring you back), whereas for ghwt you'd really have no choice other than to drop down that player a full difficulty, which might suck because then your part just becomes boring.
when you want to choose songs to play from the list in ghwt, you can arrange it either in ABC order by artist, or ABC order by song title -- those are the only two options, if i remember correctly. there are also no separators or anything of the sort so you can't skip through the list to look for particular songs very quickly. rb2 has a great menu; there are a ton of ways to list the music, and each way has a separator -- at the press of a button, you can go through each group of songs, and if you hold that button you can jump directly to a category by ABC order (eh, kinda hard to explain).
the fact that you can import pretty much all the songs from rb1 to rb2 (except . . . three?) is also a huge plus, something ghwt doesn't have. it costs $5 to do so, but that's a small price to pay for the feature.
the "heads up display" or w/e you wanna call it; on ghwt, you have a small meter with a needle pointing towards red/yellow/green depending on how well you're doing. it's not the best indicator when you're playing fast/difficult songs, kind of hard to notice and the fretboard only begins to flash red like 2 seconds before you fail. on rb2, you have a large, vertical yellow meter instead; as you begin to do worse, the meter just drops, and when it hits about 25% full the chart begins to flash red . . . much better indicator than ghwt.
ghwt's star power meter has a similar problem . . . small bulbs that begin to fill up in blue color, whereas rb2 has a horizontal yellow meter RIGHT below the chart, so you can easily tell if how much energy you have.
eh, it's 2am and i just got off work, so i'll leave it there for now.