The S50 is a narrow, chopper-looking bike with a high-performance, dual-carb motor. It has aggressive cams, and steep gearing to make it very fast off the line and in the quarter mile. Fast, agile, lots of lean angle, not the most comfortable bike in the world. Started production under the name "Intruder 800" in 1992, and the styling dates to 1986 as the Intruder 700. It is, without a doubt, the performance king of mid-sized cruisers. But the styling is dated, so it does not sell all that well anymore. Great bikes though- I put 135,000 miles on one, and have 54,000 on a second one.
The C50 is your basic fat-style cruiser. The motor is the same size as the S50's, but it is actually a completely different motor. It has fuel injection, a longer wheelbase, lower gearing, and is much heavier. Performance off the line and in the quarter mile are much less impressive than the S50's, and it does not lean as far in the corners. But it is a more comfortable bike and a much, much better on the highway than the S50. It began production in 2001 as the Intruder Volusia 800. It is comfortable, and physically large for an 800cc bike, so it has been the best selling bike in its class for several years. My girlfriend had one, and while it lacked the performance of the S50 it was a very, very nice ride and I enjoyed borrowing it. She traded it though- too fat for her to ride, as she is very tiny.
The C50T is for "Touring". It adds a windshield, saddlebags, and floorboards to make the bike better on long trips. As these are the most popular accessories for the C50, the "T" package is worth looking at.
The M50 is basically a C50 with different sheetmetal, to make it look more muscular. It is a little lighter, so performance is a bit better than the C50. It is Suzuki's newest cruiser, and the aggressive styling makes it very popular, as not everyone is in love with the fat styling of the C50.
