Overall, Rock Band's guitar solo tours have a better progression of difficulty, as in the challenge ramps up fairly steadily between difficulty levels and on each tier of songs.
On top of that, playing the drums is incredibly fun.
Like mack said, if you can get people to come over and play with you fairly often, it's definitely worth it.
As for the gear, Rock Band's Strat-like guitar has those buttons that are close to the strum area, they are a fun novelty and can be used for all hammer-ons and pull-offs during "solos" of songs, but it can be tough to adjust to their closer spacing when the notes are flying. It also has a 5-way selector switch that can be fun to play around with. During solos, the selected effect kicks in
(a spacey echo with reverb), Echo
(think standard GH starpower sound), Wah
(which simulates constantly going up and down on a wah pedal)), and Flanger
(the tone slowly modulates from scooped-mids to heavy lows to grainy highs and back down) and when you activate Overdrive
(the game's "star power") it kicks in even louder.
Use Overdrive
during a solo and you can get some nicely weird sound effects in a lot of songs.
I really don't like the RB guitar's strum bar compared to the GH Les Paul and X-plorer, but that's more of a preference thing. Also, the Strat buttons are flush with the body and simply press down, whereas the X-plorer's stick out some and push back a little, and the Les Paul's are nicely springy and can build pinkie strength well.
The drum set is solid enough, but if you are going to be playing harder difficulties and/or have people over playing a lot, I definitely recommend buying one of the metal plate reinforcements for it, because eventually the pedal
will snap at the spring point. It's not necessarily bad manufacturing, it's just a natural occurrence when you have a metal hinge pin and metal spring working against a plastic pedal.
Check
Mega Pedal for more information; the $12.95 one should be all you need.
You can use your GH guitar for a second player, and you guys can alternate bass and guitar duties. If you both choose 'Guitar' at the Band tour or quickplay set up screen, the game will randomly choose for you.
The RB microphone is fine, but if you want to try playing an instrument and singing at the same time, you can just hook an Xbox Live headset to the vocalist's controller and wear it.
The other thing Rock Band has going for it is that you make your own character/avatar, and apart from some basic customization options, you earn money by playing shows which lets you buy all sorts of new clothing and instruments. Tattoos, makeup and face paint are free.
About half of the Achievements in RB are tied to the 'Band World Tour' mode, where you have to have at least 2 people playing local and 1 of them
(their profile at least) is the designated band leader and has to be present all the time. Keep this in mind when making your first band if you plan to stick with it and go all the way to the Hall of Fame and Endless Setlist--you'll most likely want a guitarist/bassist as leader.
Woo sorry this ended up so long.
Summary: buy the whole set if friends will be over often; if not, just buy the game and use your GH guitars. Either way, definitely worth it.