I do very little "daysailing". Almost all my time on the water at least spans two days. Therefore I use the cabin almost every time I go sailing. I liked the sitting headroom in the 17 because I figured there would be days when I might be in a harbor or anchorage when the weather was especially foul. Actual sleeping space is maybe a little nicer in the 15 without the compression post as Doug pointed out, but if you aren't sleeping, the headroom in a 17 is a good thing. Also, as Bill pointed out, the 17 would be less cramped (gear+people) if you normally overnight with two. - Tod Mills, "BuscaBrisas", '87 M-17

The M-15 cabin was neat, but just way too small to consider overnighting. Good place to throw sailbags, whisker pole, cooler, toolbag, etc and picnic basket. Perhaps in the summer with a tent over the boom would work for overnighting if the weather cooperated. You'd be okay trailering and launching yourself an M-15, but get your wife on board, or take along another couple for a daysail and space is cramped, especially for overnight. - Fritz Stuneck "Blew Yonder" '74 M-17

For me, alone, my M-15 cabin is fine. With my son (8 y.o.), it's fine. With my wife, it's not fine. She finds it just too small. - Giles Morris "Umiaq" '83 M-15

Anyone who thinks the M15 is not a (reasonably) comfortable cruiser is absolutely wrong. I am 5' 8" and in my mid 50s and not as limber as I once was. My 5' 7" daughter and I have spent many a comfortable night anchored out. On the first Montgomery cruise I went on an M17 skipper had to leave unexpectedly after a few days. I offered his guest a berth in my M15 for a couple of nights and his comments after sleeping two in the M17 for two nights, and the M15 for two nights was that the M15 was more comfortable. The next year I offered a berth to Tod for a couple of nights and he was quite comfortable with the two of us. He checked out the M15 and the M17 and chose the M17 (hmmm-maybe he wasn't so comfortable.) I spent 22 nights on board the M15 the first year I had it (mostly single) and have spent 15 - 20 nights every year in the ensuing 3 yrs.
The down side of the M15 is the fact that it is a smaller boat. To sleep two in the cabin you have to move most of the gear out to the cockpit so you have to protect the gear from the dew and possible rain. The seats are angled slightly from the horizontal and they hold water (true of most sailboats) and can get the gear wet. The M15 cabin in the rain, with the hatches closed is awful. I have no extra ventilation and the inside of the cabin turns to fog in about 5 minutes. My $5 blue poly tarp saves the day most of the time but is not very elegant. The cockpit comparision between the two is obviously in favor of the M17. The M15 is a might crowded with two people but is comfortable. With three someone will be standing in the cabin. - Doug Kelch "Seas the Day" '84 M-15

I love my M15 but my Admirable thinks the cabin is too cramped for long term overnighting. The difference between the two boats is that one is a minimalist pocket cruiser and the other is a small pocket cruiser... kinda like the difference between backpacking and camping with a car full of gear, but still roughing it. - Joe Kidd "Poco A Poco" '82 M-15

My wife doesn't even care for the M-17 in this regard. With two kids on board, there's not enough room for her, she always bumps her head, and then wishes we had a bigger boat. On the M-17, you have to move slow. Fast movement results in bumps. On the M-15, you have no choice. In some respects, you bump about less in a M-15 because there's not enough room to get limbs and heads going fast.
There's another creature-comfort issue. The M-15 potty is much easier to deal with and use. The M-17 potty is very awkward to use, for both sitting and standing, especially under way. I can't believe Bob Eeg hasn't yet fixed this. But the cabin-liner definitely needs a re-design. If I were buying a new M-17, this is the only thing I'd fix. (I'd ask for other things too, but those would be customizations.)
I tend to think of the M-15 as a day-sailer. The M-17 as an overnighter. They're both camping out in in a hard-walled tent. Neither can be thought of as a home on the water, unlike my friend's Laguna 26 or your Compac 27. - John Fleming "Star Cross'd" M-17

I think you have to come to a decision as to how many nights you intend to spend on board. One or two nights at a time, and you just have to accept the limitations of an M15. If you want to go off for a week at a time, then the question of ease of life on board begins to be an important factor: i.e., if you have two days of driving rain and it's blowing like stink, the M15 cabin will take care of one person; but two people, and there is no place to move, other than lie down.
As others have said, the M17 gives you sitting head room, and more interior volume: volume goes up by the cube of the dimensions, so another 2 feet of length increase interior volume enormously: but, then you need the larger tow vehicle, etc. For single handed day sailing; one or two nights on board; with basic meals, the M15 does a great job. - Connie Benneck "Leppo" '85 M-15