The reply limit is best because you don't have to keep using them for them not being a waste. Also since it seems to easily get maxed out, it would be the only good investment looking into the future. When they release new girls, the only things you really need is items and replies. When new girls are released, the higher reply limit will help because you will already have more replies available to use. Also yes it's an idle game, so how would not increasing replies be the same? It being idle would have more incentive to increase replies because you don't check as often. There's really no point in using gems on anything else anyway and when new girls get released, any multiplier, wheel spin, double shares etc will not help you get the girls photos at all. Really the whole point of leveling the company was to unlock the girls and better chests for more items. After company 10, there's really no reason to sell your company and replies is the only logical thing to spend gems on.