I think I will prefer the 500-odd songs Rockband 2 will have when it ships.
Does that existing 500 song library really overshadow an entire community worth of DLC though? During the first few weeks I would think it definitely will...but as the editor becomes familiar to the more creative players those songs are going to start sounding better and better. If you look at the detail and nuance they have added to the editor there is a great deal of potential for not only variety but quality as well.
I think if in general terms quantity and quality of song is your top priority then GH4 is absolutely the better choice, particularly in the long run. If its specific songs you want then it could be either game depending on which songs you want.
Ideally they will both sort this crap out and use the same standard so that the choice is made extremely easy. I've said before that I really don't care which company wins this stupid war, and I really don't care who had what idea first. All I care about is which company makes the best and most fully featured game while supporting my platform.
As a wii owner that is clearly Guitar Hero and nobody can really even debate the superiority of GH for a Wii owner. With no DLC in either RB and RB2 coming several months after the 360 version Harmonix has made that choice for Wii owners....but with GH4 and all of the new features including parity between platforms its made even easier. Wii owners are pretty much bound by logic to get GH4.
Now for the 360/PS3 owners there are definitely some choices to be made and it depends, I think, largely on your current setup. If you already have GH instruments I again think GH4 is the best bet, the tricky part, and the tough decision is if you are a 360 owner with RB instruments and an investment in RB DLC. In that situation I absolutely don't blame someone for not wanting to lose those investments and frankly I don't envy the position. I'm fairly confident that by the time RB3 rolls around they will have the music editor as well and if the DLC is enough to tide you over (or perhaps you just don't care about the editor at all maybe?) in the mean time I don't blame you for waiting at all.