3520b
I got a 3520A two years ago when I upgraded from a 14" swing Delta. I've been very happy with the lathe. The sliding headstock is useful. At 35, I'm not too worried about the weight of tailstocks and such. I got the lathe because the bang for my buck was so great compared to a 16" VS lathe, only a little more.
Despite being guilty of a bit of the Tim Taylor "more power" attitude, I have found that I generally don't turn larger than 16" bowls. Part of this may be because I'm in southern California and I don't have regular access to large hardwoods, but have you ever actually held a 16" bowl? That's for holding salad for an army or popcorn for a platoon of children I don't have! If I turned a 24" bowl, I'd put a piece of glass over it and call it a coffee table!
Unless you planned to regularly turn greater than 20" diameter pieces, I think you'd be very happy with a 3520B. If I found myself turning a lot of really large pieces, I wouldn't upgrade necessarily, but I would get a bed extension and probably connect it in some way to the main bed with a spacer so that I could bring up a tailstock for really large pieces and still turn beyond the 20" swing. Below is a post about some mods Rollie Bowns made to his PM. I don't know that I'd go this route exactly, but at the same time, the issue has not come up yet.
http://www.woodturner.org/vbforum/showthread.php?t=3174
Would I love to have an even larger lathe than I do now? Maybe a Stubby or Robust? Sure, but I'd also like to live on a couple acres and not have to worry about money, spending most of my days working on developing my woodturning and woodworking skills. But neither are all that likely to happen.
I wish you the best of luck in your choices. If you are going to be at the Portland AAW symposium, they will almost certainly have a number of the larger lathes to try out and possibly sale pricing for the demo lathes (those Uhaul trailers aren't that expensive one way). When I attended Pasadena in 2003, I think Stubby and Robust were both there as well as Oneway and others. If you can wait that long.
One Addendum: The 3520B model is the refining upgrade to the 3520A. I've seen them, and they look great with some nice changes that make things just...nicer. The 4224 hasn't received a comparable upgrade as far as I know, so you're automatically dealing with older technology and an older thought process about how a lathe is used. Despite this being woodturning, the turning process & field has changed quite a bit over the last 10 years and the newer lathes reflect that adjustment.