Really the only difference is the look of the bikes. As for passenger comfort on the M50....stock leaves a lot to be desired. I think the longest day my girl could handle with the bike fully stock was about 300mi and that was with a lot of stops. Now...add a sissybar and aftermarket seat there will be no problems. We've done back to back 500mi+ days with a Saddleman Explorer, Sissybar, and she's got an airhawk as well. We only had to stop when I needed to stretch my legs or for gas/food/etc.
I've never had any issues with the bike performing with a passenger on the back. She'll still get up and go, corner with the best of them (I've scraped frame with the girl on back, she wasn't very happy about that ) and pull up hills.