I have both. Bra covers more and will last forever but ruins the looks of the bike. You also have to be careful with any plastic clips. If they aren't on just right they'll rub through the paint. Bra has to be removed every so often to clean up the tank. If grit is allowed to accumlate under there, it'll do more harm than good.
When installed right, the tankslapper is next to invisible, especially on lighter colors like silver. Once it's on you never have to remove it until it's time to replace it. I replaced the top piece this year after 10,000 miles. It wasn't worn through anywhere but had plenty of marks from my jacket zipper. Not the most durable but it does its job well.
I prefer the tankslapper.