I'd use high gloss, not because it looks any different, in this case, than any other, but high gloss will better resist corroding and chipping off because of salt water exposure. Spray paint is probably your best bet, but that may be too difficult if you tank is already set up.
What is the benefit of painting the back of your tank? For a few dollars, you can buy any color background that you want and tape it on. If you get tired of that background color, you can easily remove the paper and give your tank a "fresh" new look. Seems like a no brainer to me, but what am I missing?
it gives a cleaner look to the tank no air pockets bulges etc..
the other thing is yeah you can remove the other and change it to another color but most people dont they get a color they like leave it there and never have to worry about it again.
another side to it is that a blue or black background fills in the holes until coraline algae gets a good hold on the back glass.
most people dont clean there back glass so it helps to make it looks cleaner than it is LOL
Mike
Are you getting a lot of reflection with the satin? That might freak some fish out a bit, for future reference I would go with with krylon flat black.
Ok ok so let's see some pics!