200 Amp service is more than sufficient for a 2500 sq foot house. I have done much longer services in much larger houses without voltage drop. In fact even on the longest ones it's hard to measure any appreciable drop ConEd is too good. With 4 acre zoning in some towns I have wired some pretty big spreads.
Copper makes for a better job and may be cheaper than doing two seperate runs of aluminum. Most electricians here would use 4/0 AL. You also have the added cost of the 400 amp meter socket and panel, they are ct cabinet style and exponentially more expensive than a 200 amp setup. If you are hiring an electrician you will also have about 1/3 more labor cost with a 400 amp service.
As far as the ac units most houses here use at least two compressors for different zones. Unless they both start at the same time (unlikely) flicker should not be a problem. A properly balanced load helps this condition.
