Rock Band 3 is the Best!
I have owned and played a variety of console games from both titles, from about the time-frame of Guitar Hero III until now. I will say that my love was initially with Guitar Hero III, even when Guitar Hero World Tour and Rock Band came out. However, World Tour quickly grabbed my attention and grew to be my favorite above all other similar games. WT held it's status as my favorite until Rock Band 3 came out, It had much better features all-around, excluding only one thing: the ability to play multiple of the same instruments!
I've installed all the RB1, and RB2 songs to my X-Box, and they are now playable in my RB3 game. This is a TON of fun, and I've also bought a few of my favorite titles that were on Guitar hero, such as Lazy Eye or Numb so that I could enjoy them on RB3. I've had many friends over to play with me, and every single time we've had a blast with the variety of songs available in RB3.
To show what I've owned:
X-Box 360- Guitar Hero II, Guitar Hero III, Guitar Hero Aerosmith, Guitar Hero World Tour, Guitar Hero Warriors of Rock, Rock Band, Rock Band 2, Rock Band 3.
Wii- Guitar Hero III, Guitar Hero World Tour, Rock Band.
Playstation 3- Guitar Hero III, Guitar Hero World Tour, Guitar Hero Metallica, Guitar Hero 5.