I regularly play War Thunder and not Aces High for that one simple reason. It is free to play, and Aces High isn't. Comparing the two games is really comparing apples and oranges though. Aces High is much more of a proper simulation with proper flight modeling and historical accuracy, while war thunder comes off like an easy arcade style game. I'll just state the advantages of each game.
War Thunder advantages:
War Thunder has better graphics by a pretty wide margin. The details of both the terrain and the aircraft models are downright spectacular.
It's also a lot more instant action friendly. I can fire up a match almost instantly and they rarely take more than 15 minutes to play in arcade battle settings. This, coupled with a large active player base ensures you can get a match quickly. In Aces High I'd be hard pressed to get anything done in 15 minutes, sometimes even finding a fight at all takes longer with the relatively low number of players.
War Thunder has simplistic keyboard and mouse controls, even a small child could play it easily, and I don't need to buy or keep a big unwieldly flightstick on my desk to play it.
The scope of the game larger for War Thunder as far as planes and vehicles. It has aircraft from the early 1930's ranging up to the Korean war cutoff of 1953, including several postwar Jets. It's newly released ground mode has tons of tanks and vehicles (presumably to compete with World of Tanks).
Perhaps most importantly, to me, I don't have to maintain a monthly subscription to play War Thunder. Yes, I've paid for premium account time in the past and other micro transactions to unlock plane slots but now that I've had my account for over a year and I've unlocked most of the planeset and trained most of my crews, there's really no reason I ever have to spend money on the game again and I still retain the ability to play it as much as I want. Aces High was always hitting my credit card for 15 a month whether or not I even played it on a given month and that got old fast. It really didn't seem worthwhile if I only played it once or twice during that month.
Aces High advantages:
Aces High is by far the superior simulation, with by far the best flight modeling and damage modeling I've seen in this kind of game. War Thunder by comparison has a completely nonsensical arcade flight model, that will have bombers like an A-20 pulling 16g turns while a jet like the F-84B gets restricted to 9G. The damage modeling in war thunder is even more of a joke and varies dramatically from patch to patch. For example it's not uncommon to saddle up on a 4 engine bomber and unload all 6 of your browning m3 .50 cals into it till the belts are completely gone and have it still fly off to drop it's bombs on your base.
Aces High has huge persistent world maps with long term strategic objectives. War thunder has instant action team matches with 8-20 players per side, the spawns are in the air and so close together that a fast jet could traverse from one spawn to the other on a smaller map in around 1 minute. The smaller maps themselves are perhaps only 5 to 10 miles across.
In Aces High you must land to re-arm. In war thunder depending on the plane, weapons and crew everything reloads in the air. A heavy bomber will regenerate it's bomb load in as little as 30 seconds, which is less time than it even takes to set up a proper bombing run in most cases. .50 caliber armed planes will reload their belts in as little as 6 seconds (you can't even fire them out that fast). However, Aces High doesn't model guns overheating and jamming and you can just hold the trigger down till your guns go dry. In War Thunder the guns will start to heat up and jam if you fire them continuously for too long.
Aces High doesn't have the nonsense level up and upgrade system that War Thunder has. Every plane and vehicle in War Thunder has both a crew and several upgrade parts that have to be leveled/unlocked. These things dramatically affect the overall performance and capability of the plane and have pretty much nothing to do with reality.
In the end, I like both games, both are fun in their own way. But the one I'm still *playing* is War Thunder, simply because it's free to play and provides a fun gaming experience that can fit into a short timeframe.