I sometimes reuse the old gasket, if it is in decent condition, with a thin film silicone sealant. Otherwise, I put on a new gasket. I think silicone that holds a film like Ultra Gray or Right Stuff without a gasket would be fine too.
You are not sealing oil in, but I think it is good not to have metal to metal contact.
I'd use a gasket and no RTV or RTV and no gasket. Any RTV should be fine but Permatex Anaerobic Sealant would be my suggestion.
My only thing with sealant there is that it's very difficult to separate the trans and t-case... there are only a couple places to pry from and often I have to pound a screwdriver or putty knife in and it damages the flange. A gasket by itself IMO makes things a LOT easier to take apart next time.