I doubt it's an actual limit. More likely, that's just the biggest card they could test it with because it was the biggest available at the time. I expect larger microSD cards would work just fine with the S5.
Well the focus on the camera might be a bit better and the battery might last a little longer on the S7, but it does not have the "wow" factor over an unlocked S5 that you can put Marshmallow on. Besides, if my battery dies on the S5, I can pop in a new one. Bottom line for me: stick with the S5 until they really come out with a lot of major improvements. Holding out for the S8.
I'm in this dilemma right now, the performance boost and improved battery life over my current S6 Edge are what tempts me to get it (I managed to live without the micro-sd and the water-resitance aspects although these features are good bonuses to have, I'm not considering them that much), on the other hand the design and the looks of the phone are nearly the same although in case I go with the new Edge variant the bigger screen will comr in handy, that's why I'm hesitant to decide although all in all the S7 Edge is a better phone than the S6 Edge, I might end up getting one after all