Wow.
Just got the Rolls PM55, and it's heads and shoulders above the ART My Monitor. Couple of things:
Battery access - cool little pop-out plastic caddy for the 9-volt that comes in and out without any tools. By contrast, the ART has a flimsy piece of metal with miniscule tabs at the end secured with a non-captive philips screw. It's only a matter of time before the screw or this tab are lost.
Mono/stereo - Rolls unit has a jumper to configure TRS input as dual mono or stereo - nice. Also has a method that will switch the ground lift by sending phantom power, if needed, on the mic channel - nice.
Rolls unit has 1/4" and 1/8" headphone inputs - no adapter needed. ART only has 1/4" input, necessitating an adapter.
Bottom line - I'll be buying another 2 Rolls for our drummer and keyboard player; the ART unit is on eBay (with a free 1/8 - 1/4" headphone adapter).
Thanks to everyone for their suggestions and recommendations on this endeavor.