To be fair, I don't play the games for fun, per se. If you only play games that you'll like, you're not going to get a lot of soul gems. And if you really do like the game, you probably won't be farming SG as often, because you'll be spending more time on that game.
That being said, to answer your question, if you have War Thunder on your game task list, do that. If you got World of Tanks as well, feel free to do both and continue playing the one you like more.