Is it going to be the same physical server(s) and same pipe?
If it is then there would be no reason not to host them on the same domain.
Even using the same domain you can specify a different A Record for the galleries. Something like galleries.domain.com and have that point to a different server.
I would expect alot more traffic to the galleries that would not be productive and would therefore host them on a seperate box/pipe so it would not slow down the paying members.
Just my
