I have a mustard in with my gem, black and fowleri.
I bought him as 1" and now he is 2.5" after about 6months. He is a model citizen with the other tangs.
I've had one for a few years in the past about 10yrs ago. What I like about mustards is that they are like a sohal on crack.... Meaning they continually pace and move about, but at a faster pace. These guys come from heavy current zones and need lots of flow to be happy. They will almost seem to ride the incoming flow then within a sec be one the other side of my 12' tank.
Easily one of my all time favorite tangs for that reason alone.
Now, if you asked which one...a mustard or a fowleri. That would be a tough decision for me