I'm building up an old Trek road bike as a commuter. I want to use a triple front with a 48t big ring, like an old MTB or touring crankset. I've seen a few on ebay that should fit the bill. The problem I have is what derailleur to use with a 48/38/28 setup. Should I use a road triple der like a Sora, or a mountain der?

Road (I used Ultegra) will work fine. In this case, it's a function of the swing (# teeth on smallest ring to # teeth on biggest ring) that the der will accommodate. As you say, that 28/38/48 is a common touring setup, and most of the tourers use a road FD.

For everything you could ever want to know on the subject, we turn--of course--to Sheldon: https://www.sheldonbrown.com/derailleur.html

A road triple will work fine.I used a 105 triple with a 46/36/26 crankset. Older mtb mtb cranks use to sue 46 and 48 large rings,and derailers for them will work. Current mtb derailers for the small ring mth cranks might have a problem with the bigger rings, altho many are speced for a 48 max chainring.Road derailers will have the appropriate bottom pull for your application.

That's capacity,which is different from max chainring.

Thanks for the help, guys. Is there much difference in function between the road ders? I've got Campy on my weekend bike and I've never been able to tell the difference between the Mirage Ihave on there now and the Veloce that I had before. Is Shimano like that?

Thanks for the correction, Sydney.

In terms of functionality . . . considering $30-40 for an Ultegra triple FD or $50-80 for a DA . . . I'd go with the Ultegra. It certainly won't be the weak link in the proverbial chain.

Sora is functional,but FD are cheap,especially if you find a deal on a 105 and use a coupon. I typically use whatever falls out of the parts box and have no complaints.I don't think you gain a thing with ultegra on a comuter.