Assuming just summer camping? I think it all depends on how much you will use it and how much you want to dish out. A few years back we picked up a columbia cougar flats for a couple hundred at #^&#^&#^&#^&s. Not the top of the line by any means but definately not bottom of the barrell either. I think its 10 X 16 with divider and one side has lots of available "screen" so it has bailed us out as part screen room part tent in nasty weather (yes we had table & chairs in there
) but we were dry
Definately if car camping, go as big as you can or you will wish you did later and why not if your driving to the campsite. Backpacking is a different ballgame because remember you need to pack everything on your back wherever you go.
As far as sleeping pads go dont think you will get a decent nights sleep on an "eggcrate" alone. If just car camping the airbed types are easier on the hips and back and not bad for sleep. If willing to spring some cash or plan on doing more backpacking style camping in the future I recommend the Themarest Neoair (this goes with me on ALL camping trips now and its the bomb!)
Also second the 2 tent idea if with kids. They will feel special in their own space and the adults will have theirs.
Good luck, have fun and happy camping