Another from the UK
There is a 3rd option : You Rock Guitar
See this thread: http://www.rockband.com/forums/showthread.php?t=211852
I was lucky and picked up one for £100 from Maplin a long time ago, but a quick search seems to show they are more expensive now. Maybe you might get lucky and see one cheap on ebay or a music shop.
The advantage is you get the 22 frets instead of the Mustang 17 frets. RB3 is good for helping you see where your fingers are but RockSmith is better overall with the adaptive difficulties and handles more such as bends.
hmxhenry "We never stopped loving you, not for a second."
Gamertag: S F Booker
Pro Drums: Medium
Pro Keys/Keys: Easy/Hard
Pro Bass/Guitar: Easy/Easy
Welcome fellow brit!
I got really lucky with my mustang. Bought it from here: http://store.gameshark.net/Rock_Band_3/260. They don't have much on there at the moment, but i'd keep an eye out for one to come up.
Amazon has it in stock, but they only have 4, so act fast! It's beyond me why it's £60 more expensive on the 360 though: http://www.amazon.co.uk/Rock-Band-Wi...7220231&sr=8-1
I'd buy that and the on demand version of rock band 3. £130 for all of that is not a bad price to pay at all.
If you're looking for other pro instruments, they're out of stock of the pro drums. However, they do have keyboards that are surprisingly cheap: http://www.amazon.co.uk/Rock-Wireles...220352&sr=8-11.
Hope this helps
Edit: That keyboard bundle comes with the game. That's an absolute steal for that price!
SillyStou: What will happen to this place when Rock Band ceases to exist?
RockBandRocker: We will all migrate to the Dance Central forums. It will be like Moses leading the Israelites out of Egypt.
Munnchy: Only Flashier, with impeccable dance moves.
RockBandRocker: With more glitter and glam. We will survive