I find IL2 hard on my machine(p3 1ghz GF2MX) and have to run it at medium resolution with clouds off and even then it runs slow.AH runs very smooth for my setup.
Damage modelling in IL2 is undermodelled like AH is overmodelled.I can live with AH uber cannons but IL2 needs to be adjusted.
Flight modelling has IL2 in a slight lead(needs compression in dives for 109's and flap damage at high speeds)Spins are what IL2 has on AH,same with challenging landings and takeoffs.
Tracer effects are cooler in IL2,looks just like war footage and is really hard to learn.
Graphics AH wins for me cause I can run it at high res.IL2 looks great up close(screen shots) but has a dullness at range.IL2 has the greatest sun effects ever modelled.
Online flying a boxed sim is a different experience altogether.Right now AH is the undisputed king of arena flying.IL2 IMHO has a ways to go before it unperches the king of boxed sims(EAW).That was flawed too when it was released but it has an X factor that spawned many great squads and perhaps my favorite WW2 combat memories.(Squad wars..ladder play)
AI IL2 is not great..EAW,BoB,Mig Alley are all better.AH AI on the otherhand has set a new standard with the release and subsequent updates of Fester-Bot..

Overall I would not try to comare it to arena flying but to other winner boxed sims that can run co-op missions..<S!> and Welcome to AH..
