The main difference between the two universities is clear with a quick glance at each of their websites: Cambridge don't explicitly offer Electrical Engineering, or any other discipline for that matter. Instead, all engineering first years are in the same classes for at least the first while, after which time people start to specialise. By the end of four years, you (hopefully) have a fully specialised degree in a particular discipline.
Imperial's system is far simpler. Different departments for each discipline and you choose on your UCAS form and follow that route.
Now, the question of which is better is clearly a close call. Naturally, each university must think their way is best, otherwise they would do it the other way! Personally, I'd favour Cambridge's system as, I feel, a gradual specialization means that even the civil engineers have done a little Electronics (and vice versa) giving you a more useful education. On the other hand, you can quite easily argue that Imperial's immediate specialization allows for a better course content.
Of course, no one has done both degrees so we can never truly know...perhaps someone should try it to put this argument to rest...after all, it does seem to come up every week!
EDIT: Assuming the OP is applying for 2009 entry, can't you just apply to both (like I did) and see if you are lucky enough to have to make the decision in January? I, however, had it made for me...but you can hardly be upset either way.