I haven't got or had the 3175 but I have bought the 4819 fire station, I just haven't had time to build it up as yet (although Diane does know about it). I've also got the parts to extend it to accommodate three fire engines but instead of getting the DS add-on I just bought the bits I wanted, missing out some pieces which I felt were unnecessary and actually in my view extended the fire station in a less useful way as each extension is staggered behind the previous one rather than being directly alongside it. In addition, it was cheaper doing it this way!
As regards the DS add-on for the new fire station fitting the old one (or adding the parts I did), there should be no problem so long as there is a System-X support with holes for connectors on the side to which you're wanting to add the extension.
Although I haven't built the 4819 yet, looking at the two sets on playmoDB, I'd say they're about equal for space in the centre section (and maybe the 4819 is better here), but the 3175 has the little extension on the left. Against that, the 3175 doesn't have a hose drying/drill tower which the 4819 does. I intend to add a small extension to my 4819 on the side of the appliance bays which will act as a workshop/store area.