The difference between them is the battery size. The bigger battery was the previous model. You get a few more miles with the larger capacity battery. However, that battery exceed airline regulations. For handicap people, the battery capacity on this MAY allow it to travel. Check with your airline.
I’ve had the original Segway Minipro which is probably model 320 (since this also gets 14 miles per description) for a year-and-a-half with almost 1,000 miles on it. It’s a great and relaxing item and I use it several times a week just for pure enjoyment or to cool off after working out. $350 is a good deal if the unit is in near-new condition (and the battery as well) since replacement batteries run about $250. The the alternative batteries I believe were taken off the market since the machine has difficulty reading non-original batteries and it affects the maximum speed, according to my online research. I did have to replace the tires after a year due to weakening from being compulsive with keeping them at the 45 pound recommended pressure despite the fact that the tires are rated at 30 lb. Might decide to pick one up t this price to have spare parts, a spare battery, and a separate unit to try out with friends. I own a spare handle for people that want to steer with their hands although I prefer the knee control myself.
I have had mine for a year now and its great for me holds a charge for a long time and is great for pushing the kids around in their non motorized cars