I have an upright in the living room. It's a pretty open room, hardwood floors, and I do have it placed near an inside wall but that's just because where it looked/sounded best. My house was constructed in 1990's, and most modern homes with reasonable insulation probably make the "inside vs outside wall" argument kind of moot. I would definitely first check to make sure any placement in those rooms didn't have a major issue - near a draft, in direct sunlight, over a heat duct etc...
Other than that I think you need to decide how the piano will be used. If you regularly are going to have parties and such with people over, then the living room would probably be best for your audience.
After those considerations, I would decide based on acoustics. I'm confused by your statement about echo in the living room, as mentioned if you have "stuff" in it that will help subdue the echo. In general more space has the opportunity for better overall acoustics. Your mileage will vary though, and don't be so set on where you put the piano. I mentioned earlier that mine is on an inside wall, I moved it from an outside wall because 1) it did look better which got me thinking in the first place and 2) it sounded better. It sounded even better when I moved it again from about 2" from the wall to about 8", and moved it 6" further away from a corner and put some sound panels in that corner. It's amazing the difference small changes in placement can make. I have an institutional piano with big casters and super easy to move around on hardwood, so I can make those types of adjustments.
Based on very little info that you gave I'm going to say living room because of the size. Echo can usually be dealt with pretty easily with carpets, drapes, panels etc..
Good luck.