I lean toward the upper hand going to the attackers (once a major/minor fleet battle is won). One of the big problems is (IMO) mines and other defenses, they are not enough to stop a fleet (once a fleet has a few good sweepers, or lots of crappy ones, mines become useless). Sats are still a very MINOR defense and fighters are useless mid-late game (as are missile ships/defenses once good PDC is researched). The only times I have seen the defender come back is with a sudden discovery of the Talisman, then it is a matter of holding out till a fleet can be built. That and planets are waayy to easy to 'glass'. (well, ain't I positive today
).
Don't get me wrong, this is possibly the best 4X game I have seen, just a few things about 'balance' in weapons and defenses need to happen (that and the AI needs to be taught to 'remember' what happened the previous turn
)