I don't know that I have a preference. Whichever side is convenient for the reason I'm mounting/dismounting. All else being equal, I probably use the left side more.
Either way, I have to swing a leg up and over the steering wheel. The seating layout on my '88 JD750, tho comfortable while operating, does not work especially well for my 6 foot 200 pound frame.
I love absolutely everything else about the tractor though. It has been through **** with me for 25 years now and never had a serious breakdown.