Have you gone to their websites and looked at the pictures?
If you haven't, that might help (there are links to the companies in the disABILITIES FAQs thread near the top of this board).
They both have regular and heavy weight
ECVs (the weight might vary depending on the model of
ECV the company uses). The heavier weight ones have bigger batteries. larger seats, etc to be comfortable and have enough power for a larger person. That's the difference in the weights.
All ECVs drive pretty much the same. Most people find the 4 wheel model, like you can rent at the parks to be a little more challenge to drive. Because they are larger and have 4 wheels, it takes a bit more space to turn them. The smaller 3 wheeled versions are easier.
What a lot of new users do is go to a store like a grocery store, Target or
Walmart and try out the ECVs they have for guests to use. Those are MUCH bigger (and also have a cart attached on the front), so if you can drive one of those, anything you can rent at WDW will seem small and easy to drive.
ALL of the companies listed in the disABILITIES FAQs thread are ones that have been recommended by DIS posters repeatedly over the years. I think you would be pleased with any of the companies.
Randy's is a good choice for a new user, who is a bit unsure. The other companies deliver to Bell Services or the front desk of your resort. Randy's delivers right to your room, so you have the opportunity to get a demonstration and get all your questions answered. Some new users do feel a bit more confident with that personal connection.