My opinion-
If I had to pick between bigger or smaller, I'd go smaller. Some people will say go bigger, and trim. I personally like them more ch better when they are not trimmed. Also, keep in mind, the sizing is based on the width of your 3 fingers. I shoot a Med/Lg glove, and a small tab. I even like the XS tab a lot.
Great for hunting, no disadvantage at all.
IYou can certainly wear a glove under the tab in cold weather, just make sure you practice a little that way. I hunt barehanded, with a tab, no matter how cold. I just rest my hands in a hand warmer that I wear around my waist.
Those slits in the BW shouldn't bother you at all. I don't think you will even notice. I shoot split finger, love the BW tab, but have been considering a Bateman myself just to get cordovan.