We were originally interested in the M-18, but the couch would definitely not comfortably fit an adult over five feet in height. This was the primary reason for choosing the M-22. We did not require the bunks, although they make great storage areas. The M-18 could not be used for more than two adults and two children. The couch bed in the M-22 is much larger, and can be used by two adults, and the bunks are also useable for adults as well. The M-22 is a bit longer than I would have liked, but we can take a couple of adult friends with us. We had no issues with the M-22 in the first year, and found it to be a very well constructed trailer.
Cam