The Airism shirts are very form fitting unless you grossly oversize them. I would just go TTS and not worry about tucking them in, to be honest.
I generally like the two that I have, but I've noticed a few things. While they're extremely comfortable, they do have a tendency to make me sweat a little harder which can keep me cool but is also pretty unpleasant. The roundneck ones also tend to ride up my neck a little higher which is irritating, I'd probably go for a v-neck one in future just so that it doesn't show on my neck underneath my shirt, but maybe your body shape wouldn't have this problem.
Lastly, I've had them for about 6 months now, and both of the shirts have stretched out in the washing quite a bit and are starting to hang loose in a couple of places, which personally is not what I really want in an undershirt.
I generally like the two that I have, but I've noticed a few things. While they're extremely comfortable, they do have a tendency to make me sweat a little harder which can keep me cool but is also pretty unpleasant. The roundneck ones also tend to ride up my neck a little higher which is irritating, I'd probably go for a v-neck one in future just so that it doesn't show on my neck underneath my shirt, but maybe your body shape wouldn't have this problem.
Lastly, I've had them for about 6 months now, and both of the shirts have stretched out in the washing quite a bit and are starting to hang loose in a couple of places, which personally is not what I really want in an undershirt.