Sailfin is definitely too big for your situation.

I too have had poor luck with blue tangs. I've likely given up on them for good. It is so disheartening to go through more than one of the same species like that. I am going to go with a Naso for my next Tang I think, but I have a 125 and only 3 other fish. I think a 50 would be small for a Naso.

If you definitely want a tang, I would go for a yellow- realizing he will outgrow what you currently have eventually. I've had pretty good luck with yellows, and they are very readily available and tend to be fairly hardy usually. Also, you can't beat the bright yellow color in the tank, they really pop out. Start with a smaller to medium sized one. Down the line a yellow would eventually outgrow a 50gal, definitely, but you can enjoy him for a while, and then upgrade to a bigger tank later or trade him to someone with a bigger tank.

Btw, remind the wife that one of the characters in Finding Nemo is a yellow tang if that helps your cause any ;)