I have not mixed and matched the Sonance and Monoprice speakers to A-B them, but I ran 12 pairs of Sonance 632 speakers in my last home. They certainly sounded good, but I ended up putting Monoprice in my basement setup and a couple of other rooms and I thought that the 8" in-ceiling speakers sounded more full and rich compared to the Sonance speakers. I never played them at high volume, but would enjoy them while working around the house. I find their 8" 3-way in-wall speakers to be the best they have, and to sound better than any 6.5" two-way speaker I've heard.
I have used those for my clients, with statements along the lines of "Shouldn't I pay more?", and then had them smiling after an install and satisfied.
I mean, there are really good in-wall and in-ceiling speakers out there, but not at $300. You go to a Martin Logan Edge series speaker, and you are talking about a serious improvement in overall quality (better build a good wall!)...
I just installed a set of those behind an acoustically transparent screen and was happily blown away with the quality delivered. But, a boxless 6.5" or 8" ceiling speaker? You just aren't going to get much more than a typical speaker with rubber surround and call it a day whether it is B&W at $500 a pair, Polk at $120 a pair, or Monoprice at $80 a pair. Sonance delivers one of the best mid-priced models with their 831/832 series of in-ceiling speakers, but because I tend not to crank in-ceiling speakers, the premium has never been worth it to me.
If you are curious, you can certainly buy one pair, install them, then see if they meet your needs. Perhaps buy one pair on Sonance speakers to see if they offer as much of an improvement to justify buying more of them for your home.
I do agree that this is a problem, and you may get some better responses just asking in the 'speaker' area in case someone has performed a true shootout of some different models (including monoprice). I expect that I will pick up a pair of Monoprice and some Sonance speakers at some point and put them side-by-side on the same amp in my basement when I finish it to act as a 'showroom' setup for people to come by and compare. Depends on how much extra funds I have to do that in a few years.