A top university has many advantages. Better universities provide better Professors, better academic environment, better scientific connection, better equipments, better laboratories, they can get better grants, they can give better scholarship, etc. You can find better friends and get better reputation.
@aeismail has very good points on the reason you may or may not stay in the same university for all your academic carrier.
Some universities has large department and so many professors. there is a chance you don't even know them yet after 6 years being there. They work on different fields and you can work on different field if you want. but that's not true for all universities.
Despite all of this, consider that you will carry your university names forever in your CV! This will affect your feature job opportunity a lot, in industry and mainly in academic world.
But as one my professor says: "sky is the same color everywhere" ;)
All said I suggest you going to a better university. At least try to.