When Kubota updated to the 60 series, they re-adjusted the hydro trans to be less jerky. Small touches here and there in improvements, but most obvious was the return to metal rear fenders.
The differences are not huge, the jerky HST versus the "roll a bit" HST was big!
I had a BX2350 and several other models of B's and BX's for reference. I traded the BX2350 very soon after buying it for the BX2660 because of the jerky transmission in reverse on the BX2350. Also the seat safety cutoff switch on the BX2350 was so bad it would constantly cut off when my wife drove it and for me when going down hill on a hill side property. Seat cut off issue did have an easy fix. The hood didn't fit close on the BX2350 but that wasn't an issue for me, didn't care metal vs plastic.
Unless a big discount I'd never buy a BX2350 unless the property was flat and needed to rarely use reverse. Straight ahead mowing on flat land the BX2350 is fine.
You hear it all the time, but do a search on this forum. loads of information and debate on this exact subject. But for a quick summary... Go with the BXxx60 series if at all possible and you will be the happiest