To behonest if you want to be competative then you are going to have to choose one or the other.
At low level and small shows it will not matter do both, at bigger shows and higher levels it will matter a lot so choose one do high level in that and then stick to the lower levels in the other.
Stan was an M&M, we did showing where he has to have a full mane and tail but his tail was feathered to a point just below his hocks. He was also capable of medium level dressage, which over here is where is gets very very serious and you need a double bridle, for affiliated dressage you have to plait (you can get chucked out if you don't) over here and tails tend to be long. So we chose to do high level showing, turn out appropriately for that class and stick to unaffiliated dressage.