The difference between Guitar Hero and Rock Band is the addition of added instruments (Bass, Drum and Microphone) and it is a welcome one.
I never actually took to Guitar Hero that much, but nightly with Rock Band we have family "Jam" sessions and I have to say we are getting pretty good.
Rock Band has been a great bonding experience by allowing all members of the our family to participate at once. My family consists of three people, but Rock Band will accomodate up to four people at the same time.
It is also possible to use your Guitar Hero guitar with Rock Band, which is helpful.
The Rock Band bundle package comes with a guitar, complete drum set (4 drum pads and drummer sticks) and microphone.
The song selection is small compared to Guitar Hero, but there is still an ecletic selection to choose from and Xbox 360 has other songs available for download when your ready or just too good at the ones from the game.
My biggest pet peeve would probably be that the set up takes up a lot of my living room space, it has inspired my to make the extra room into a game room though so things are balancing out in that aspect.
It is a blast to see my eight year old and his dad rocking out and enjoying the experience - and fact be known...I'm hooked too.