I played WB since the first pay version (post-beta) and I still have an account. I have been flying AH since beta. I'm part of Northolt Wing which fields active squadrons in both sims.
Here's how I see it:
AH is pretty much a continuation of the original spirit of WB, with a lot of new stuff added in. Most of the stuff we asked for to improve WB 2.0 and WB 2.01 is here.
IMHO WB releases 2.6 and above were simply BROKEN. WB has changed hands a few times and the different owners and development teams have tried to repair the damage and start in a new direction. But I'm not sure that they know where they are headed, except to copy new features of AH. The best thing about WB is the scenario community, but they haven't had a full scale scenario in years, and the AH TOD series is on par with the WB S3 series now.
WB3 AdvantagesDamage Model (finite element analysis etc)
Graphics Engine (Looks better and seems to perform better on slower machines)
Player-Developed Aircraft (should give it a nice planeset growth rate)
Custom Aircraft Skins
More Educated Community (IMHO)
AH AdvantagesGunnery Model (Every bullet is modeled, with recoil, airframe vibration, and dispersion)
Engine/Prop Model (decent model of variable-speed superchargers and constant speed prop)
Weather
Gun Camera Film Recorder/Viewer
Built-In Voice Comms
3D Sound (sounds incredible on 5.1 system)
Custom Nose Art
More Choices of Planes/Vehicles
Terrain Editor and Player Developed Terrains (dozens of them now)
Active Head-to-Head Community
More Detailed Sea/Air/Land Strategy System (Player Controlled CV's, Trucking Depots, HQ, Many Factories, Train Stations, Truck and Ship Convoys, Trains, Towns)
Audio 6-Call Button
More Polished and Complete Product
More Diverse and Humorous Community (IMHO)
Things That Are Different But Not Clear AdvantagesAH Controls More Responsive
WB Controls More Sluggish
Radar Modes (AH has mixture of in-flight "dot" radar and in-flight "sector counter" radar depending on the arena. WB3 system is a little different.)
WB has some planes from 1940, AH has none.
AH has several "late war wonder planes" and WB has only 2 or 3.
AH view system more flexible, WB system less flexible but possibly more realistic.
Both Communities are some ugly drunk mothers. I've been to the Cons, trust me.
They still are not terribly different products in the grand scheme of things. Supposedly WB3 pricing will go up to $25/month soon. Compared to the $2/hr days they are both impossibly cheap. Try them both and decide for yourself.