I have one of the XL T*utto for my machines. I wanted to load machines from the top and not from the front of bag. The wheels roll and swivel which is a great feature when moving it. Some bags only have two wheels that roll "one"direction. They aren't as easy to maneuver.
Are you looking at serger bags or machine bags?
It is harder to load a machine in the front of bag over loading the machine from the top. For me, it would depend on the weight of the machine. These are nice bags and I would order the larger size to have room for a few more supplies. I added a layer of 2 inch foam to the bottom for extra cushion in the bag to protect against bumps.
Check your manual for the specs and your measurements then compare against the size chart.
http://store.tutto.com/serger-on-wheels-size-chart/