Empire Poker and Party Poker are what I like to call "different doors" into the same room. I suspect Empire Poker has some kind of sublicense from Party Poker but have no knowledge of this. Will ask around next month when I go to a tournament and see what I can find out.
This "same room" results in them sharing tables and players, as You correctly point out topspinelite. Consequently they have the same number of players at any time.
To answer the "any better" question, the only thing you are then looking for is extras and not the games themselves.
I preferred to play at Party Poker for the 10% monthly bonus for players who signed up early but that is being discontinued after this month.
You might wish to sign on with Empire Poker for their bonus and any extra perks they offer. The ecash processors are the same and their cash out procedures identical.