Definitely go for the upgrade... when i upgraded from my 21" to my first commercial mower I saved an incredible amount of time(obviously) and my business keeps growing and growing. Furthermore, people have a tendency to take your 'business', or services, more seriously and because of that I got a lot of new clients!
As for a mower, if you can wait until the end of this mowing season you can get a MUCH better deal. I got my 36" Toro for $2100.00 at the end of last mowing season. It's a gear driven machine (cheaper) but it sports the new T-bar system, i'm pretty sure it's on all, or most, makes now and is a helluva lot easier than learning to handle a commercial mower plus the pistol grip set-up.
I'm not quite sure if you need a sulkie though... if your accounts are that small than it really isn't necessary, but Toro does have a nice tru-trak sulky...
Well, good luck and I think if you upgrade you will never want to look back!!! (sorry for the breathy reply)
Lawns By Ryan