FS:SDOE does allow you to design your own skins, you can also design new planes, tanks, airfields, shore forts, terrains etc.
It allows you to build a mission involving up to 16 aircraft and then go online and let other players take over any of the aircraft with the computer controlling the others. For example you can tell 4 Lancasters to take off from Kent and fly to France and drop their bombs on an airfield via selected waypoints and tell 2 P51s to escort them while 4 Spitfires sweep over France. Meanwhile 6 FW190s are ordered to take off and patrol over the field and mobile ack and shore forts wait to shoot at any allied aircraft. As I said you may have 16 players taking all positions or one or two of you could just fly the P51s and let the computer handle the rest. If you enable it you could also leap from plane to plane as the fancy takes you though this can be disabled for competetive play. Last night I flew a mission involving a US army convoy broken down on a bridge. Two of us took FW190s with rockets to attack while two P51s took off from a nearby field to defend and MTBs sailed up the river to provide AA. Great fun!
As I said , you are only allowed 16 players at one time but Warbirds only allows 32 in any one part of the arena and has nowhere near the graphic detail. Aces High does not seem to be quite as detailed in terms of scenery either but the trade-off with FS:SDOE is that the terrains cover quite a small area eg Kent & NW France which is a little too small to put together an atmoshperic one hour mission like you can do in Warbirds and presumably will do in Aces High.
A good site for FS:SDOE is
www.fshangar.com Rob