Theoretically stable frametimes should be better for aiming due to it being consistent but I don't have any scientific proof for it, you also have to consider that the overall responsiveness could be (slightly) lower compared to uncapped however as long as the framerate is close to 240 I imagine that it should be fairly marginal on 240hz , compared to 144hz anyway. I think Battlenonsense referred to some test/study in his video on frametimes so that might be something to look into.
But honestly I don't really know much about 240hz, on 144hz I definitely prefer uncapped over 140fps + Gsync on 144hz 1440p just because it feels more responsive, I've been wanting to try 240hz but I wouldn't really want to trade my 1440p for 1080p and there's no way I'd go with 1440p 240hz since 144fps is already hard enough to achieve. 240hz + Variable refresh sync seems like it would be pretty nice though.