DCS World is a free download (on this months PC Pilot too) and includes an SU-25 frogfoot. The other DCS sims you can buy (A-10C, P-51, LOMAC 2) all then plug into it.
I have the Mustang plugin and think it is brilliant but it is a full on sim and getting the engine started and the aircraft off the ground is a lot harder than in il-2. The downsides is that there are no other WW2 aircraft available and if you want to engage ground targets they are modern too. This poses some interesting challenges like how to use cloud cover to position your self in order to attack gunship helicopters armed with A2A missiles...
Cliffs of Dover is OK now that it is patched, especially if you use some of the on line guides to configure it properly for your PC. My main gripe is that the AI is very poor and dog-fights get boring.
You still can't beat the re-works of Falcon 4 if fast jet combat is your thing and DCS does not have want you want or your PC is getting old. Falcon BMS is the one to go for.
I just wish I had more time for these.