Dave - I use all three, DPM, Rix, and Smalltown; and I am pleased with all of them. As Chet said, painting all of the windows can be a pain, but I don't try to get in a hurry and maybe paint two walls today, and put it down and paint two walls tomorrow. That said, even the Walther's Merchants Row buildings have molded in windows, just many more of them. These kits are relatively easy to construct, and can be easily customized to fit any need. I don't always use them as their name indicates.
The Smalltown kits mostly use many of the same side and rear walls between them. Some side walls have windows, some don't. The same rear walls are not a big deal unless you have an alley with all rear walls facing an aisle. All of them have window/curtain sheets now, they didn't when I first started using them. I scan and store them on the computer and use some of them on other kits without similar sheets.
The DPM kits are all unique in that no common parts are used.
The only Rix dwelling kits that I am aware of are the Maxwell Ave houses that are all the same with different porches and orientations. I use a lot of their grain bins and other accessories as well.
I need lots of structures and I have multiples of many of the DPM and Smalltown kits. I use different paint schemes and provide my own business signs to differentiate them. I also put them on different levels on different aisles so it is not very apparent.
DPM is now owned by Woodland Scenics. Rix, Smalltown and Pikestuff are all now owned by the same company, Rix, but they retain the individual branding.
Unlike Chet, I haven't found any cross-brand duplication, yet. Give me a day or so and I will post some pictures.
Willie