I'm a new guy to the DD game. By new i mean i haven't gotten into it yet. I'm in the process of hunting down a pre ban (re: old for you free staters) receiver to build an ar out of. I want to get an m203 to mount to it.

My question is two fold. Is barrel mount or rail mount better? And what rails will hold up to an m203 mounted to it?

A lot of guys like the rail mounted because you are not as restricted by barrel length. You can also run a rail mount on a non M16/M4 weapon platform.  Most rails will hold up to it, if you have something like a LMT MRP upper you will be good to go. Most FF handguards with fixed bottom rails (i.e. larue, Old KAC FF non urx, DD that kind of stuff). I however like the standard mount with (if you can get one) the DD RIS II because it lets you mount the 203 while still having a FF barrel. It just seems like a better mount up. This is all assuming you can have a Destructive Device in your state.