Caveat: I received an offer from Deloitte post-MBA but never worked there; a number of my friends and current colleagues have gone there however.
Your question: How is the experience different from MBB? From what I understand, you will do less high level strategy stuff. The typical MBB client is a senior executive (C Suite), whereas the typical Deloitte client is probably closer to Director/VP. I know, this is a broad generalization and Deloitte in particular is trying hard to go up the value chain. In spite of much lower billing rates, they still lose out to MBB however, so that leads me to think the quality and reputation of their work isn't quite on par (yet?).
The second part of your question is re. how easy it would be for you to join MBB in the future. A friend of mine did 2 years at Infosys, which helped him then get an interview at Strategy& (ex-Booz) where he's now been for 4-5 years (currently a Principal). Would he have had the Strategy& offer without his Infosys experience? He doesn't believe so. When I interviewed at BCG, another applicant was a Booz consultant trying to switch too. I also have a number of friends who did boutique or tier2 consulting pre-MBA and joined MBB post-MBB. All this to say, yes I believe your Deloitte experience would help you get an interview at MBB a couple of years down the road. There is obviously no guarantee however, and you would still have to go through the same interview process that everyone else goes through. You should also expect to start from 0 again (i.e., be a 1st year consultant when you join MBB).
Does this help? Let me know. Oh, and congrats on getting an offer at Deloitte, I have never heard anyone talk bad about this company.