RB2 will be released in December this year. It has DLC (will also include ALL songs previously only available for the first Rock Band on Xbox360 and PS3), create a rocker etc as well.
There are a few things you have to keep in mind:
- Which drum set do you like best? The GH:WT set is more realistic by looks, but it has quite a few bugs atm. Some people prefer the rock band drum set. I can't tell, since I've only tried the Rock Band drum set. It makes a lot of noise though, but this can easily be reduced by pads.
- Which track list do you like best? Keep in mind that with Rock Band, the 3 track packs AND Rock Band 2 next month, you'll have way more songs. Then again, if you prefer playing guitar over everything else, you might want to have a look into Guitar Hero 3. This game has many custom version floating around the internet. If you have a Wii capable of running these, you might want to think twice.
Rock Band 2 will have everything Guitar Hero has, MINUS the studio and Mii Freeplay (as far as I know).
The Rock Band instruments have had a big price reduction last week though, so yeah. In the end you'll have to figure out which one suits you best.
EDIT: To answer your questions directly:
1- Which one has better songs?
Your opinion matters most here. These are the tracks for
ROCK BAND:
Original Rock Band track:
http://en.wikipedia.org/wiki/List_of_songs_in_Rock_Band
Rock Band track packs:
http://en.wikipedia.org/wiki/Rock_Band_Track_Packs
DLC (available after the release of RB2):
http://en.wikipedia.org/wiki/List_of_downl...ock_Band_series
Rock Band 2 tracks:
http://en.wikipedia.org/wiki/List_of_songs_in_Rock_Band_2
GUITAR HERO:
Guitar Hero World Tour tracklist + DLC:
http://en.wikipedia.org/wiki/List_of_songs...Hero_World_Tour
Guitar Hero III (exclude the DLC songs, those will remain unavailable for Wii):
http://en.wikipedia.org/wiki/List_of_songs...Guitar_Hero_III
For custom versions, browse google, torrents and newsgroups
2- Do any of them have DLC? If so, how much does a song cost?
Guitar Hero: World Tour and Rock Band 2 both have DLC. Rock Band one does not. I don't know how much they'll cost, but according to the person above, they're approx 200 wii points.
3- Are GH:WT and Rock Band controllers cross compatible?
Unfortunately, no. Not on the Wii.