I've never used a double barrel smoker, but have done lots of cooks on my UDS. In terms of fuel efficiency, I understand the double barrel smokers are fuel hogs, where the UDS is not only very fuel efficient, but is pretty much set and forget.
I'd go with 2 two rack UDS for your purposes.
Modded Brinkmann Gourmet. Basic UDS. Rescued 22.5" OTS. SJS Mini-WSM. Stubbs. B&B Oak Lump.
Cannondale 2009 F8 (for burning off all the great BBQ)