The Axiom factory has burst at the seams with orders and all the staff is working around the clock to get everyone their gear. Apparently Ian is surviving on 1.5 hours of sleep a day but he somehow still found time to read your post and make you a generous offer. Wow!
I've discovered the M3, M5, M50 and M100v4 are all problematic in smaller rooms due to their inaccurate bass response. The M50 is the best of the bunch in this regard. All the others have prodigious bass output that is difficult to tame. The M50 is prodigious also but it appears to be more linear. I've tried port plugs but didn't like the sound. I had to cut my bass by as much as 16dB and even that wasn't enough when playing loud.
The actives are a completely different animal. They are worth the price for the bass alone. For music, I will take the actives in 2.0 over my M100/800/dual 500s. You can't possibly understand this until you hear it. In the same smaller rooms where I had bass problems with all the others, I drop the actives in and there are zero problems. No "room EQ" needed of any kind. The amplitude response curves tell the story and I would have never believed how much difference those curves make without hearing the actives myself.
So back to your question, the M5s are more accurate than the M3s in every possible way. You will hear things with them that are masked by the M3s. If you place them along the 10 foot width, you will have a wider soundstage with more images that are better isolated relative to the M3s. If someone took my M100s away and I was left with M5s in my living room, I would not complain. I thought at one time I wouldn't complain about swapping M5s for M100s in my main space but I've come to enjoy the more expansive images the M100s cast in my 4200 cu. ft. room.
