Warren...You've really got your 'homework' cut out for you if you want to upgrade from a Jet 1015VS... I too, like many others, have recently upgraded... Things you might want to take into consideration is 'total cost'... Just the cost of the lathe is only a partial cost of an upgrade... i.e.; going from a small Jet 1015VS 110VAC requirement to a larger lathe may require a 220VAC electrical requirement...consider lathe footprint...the 1015 is a small, bench top size lathe while many larger lathes require much more room. Don't forget the larger spindle size (i.e.; 1" X 8TIP vs 1 1/4" X 8TPI), which requires different chuck and accessory adapters at the very least, plus larger tool rests (i.e.; 5/8" vice 1" posts and larger heights...), etc., etc., etc. I don't mean to dissuade you and your new beauty, but these costly upgrades also add to the cost of a major lathe size upgrade...
Being only a hobbiest, I did my research over a year or so, and decided on a 'mid-size/cost' Jet 1640EVS... I took Jet's excellent C/S, warranty, and replacement parts into consideration, plus my past Jet experience...cost (a MAJOR consideration...) plus electrical power considerations. I've probably spent an additional $400 - $500 on new accessories (chuck adapters, spindle adapters, tool rests, etc.), but knew what I was walking into before I made my decision. Am I totally satisfied, YES!!! Couldn't be happier other than wishing I had the space to keep my previous Jet 1221VS as a second lathe... The fellow who bought it got a great deal and many like new accessories, etc. Have fun making your decision, but check out what's out there and your TOTAL budget before making your decision! Safe turning to you always!