howdy. it depends on several things. what is your price range? how much bigger are you looking to step up? you want to do mainly short rides or long rides? ride alone or with a passenger? buying new or used?
the M50 is comparable to the V Star 650 so if you just want to step up one or two places, then you're looking at either the V Star 950 or 1100. the 1100 is not made anymore, so if you want a new bike then you might be able to find a previous year 1100 that a dealer hasn't managed to move yet. or you can step up further to the 1300. the 1300 has become the new 'medium' bike among the V Star family in terms of price and size and specs. i would advise you to go to a dealership and just sit on some of them and see which one feels and looks good to you in person. go from there.