I have both.
I do find a curved trigger to be more comfortable, but all of my remaining flat triggers are in rifles - hunting rifles. Since it is really only necessary to touch the trigger for a few seconds (even in extreme circumstances) to fill a tag, and my 'flat' triggers are all clean, crisp, and predictable, the shape of the trigger 'bow' isn't really a factor.
Even if I watch an animal, or entire herd, for two, three, or fifteen minutes while prone or sitting behind the rifle, my finger doesn't touch the trigger until it's time to take a life.
Handguns ... curved trigger. I don't remember what it was, but I had a semi-auto pistol with a flat trigger bow and have handled a couple others. Long trigger pulls - such as with striker designs with no pre-load, or DA pulls - don't feel very precise or repeatable with a flat trigger.