Excellent point on the wheel spacers working better on the larger deck.
This tread is fairly old (2011), but I'll post a reply anyway, considering the topic discussed is still relevant four years later and that a comment might be useful to somebody.
GR2120 ( I've owned one for three years now, bought brand new) is a high-end riding lawn mower, diesel engine, 4WD and a lot of traction. Works perfectly on steep inclines I could not mow with a 2WD. Glide steer prevent any damage to the turf. It is lightweight (900 pounds with mower, 800 without). It can easily tow its own weight. Gear and transmission casings are cast aluminum. Ground clearance is modest, especially under the rear axle. Can accomodate a limited number of appliances. Personally, I only use a sturdy trailer at summer time. To clear snow, I acquired a Honda HSS1332, track, perfectly suited for the job.
BX series are compact tractors, diesel powered, 4WD, PTO, hydraulic outlets, can accomodate a large number of powered or non-powered appliances. Built is accordingly sturdy with heavy geared transmissions encased in cast iron. They can be fitted with mowers as well. Weighs around 1,500 pounds without any appliance. BUT: mowing is not what they are first designed to do. In tight spots and on short turns,
one wheel will not turn at the proper speed and will scrape the grass while slightly spinning.
A neighbour came about with his JD 4WD compact tractor to help me out with heavier tasks. As he turned on the turf, his machine just pulled off the grass from the front wheels. Turf was not destroyed entirely, but traces remained apparent for a couple of weeks.
There is no way to compare those two series: they are different machines primarily designed for different purposes. You have to properly assess your needs and priority before buying. I just outlined the results you will get with either one of them.
Raber