Drums are by far the most like the real thing. I've just bought a drum kit after playing Rock Band, and I've been able to just dive straight in to playing basic beats. Even adding the hi-hat control on the left foot wasn't that difficult to come to terms with. Obviously, I have a long way to go with the drums, but it certainly helps you out with basic coordination.
As for guitar. I've played guitar for 13 years, and it's nothing like Rock band. It does mimic the feeling of playing the guitar. You get the same buzz when you pull off a complicated series of notes as you do when you nail that killer lick you've been practising for weeks on the real thing.