Having both I can attest to the versatility of Belgians and Quarters. Mine are all expected to do anything I ask and to the best of their ability so that means riding, driving, trails, cross country, arena work including dressage. They each have one or two things they excel at but are competent at several other disciplines. In my experience working at a barn in PA individual Arabians have excelled at more diverse disciplines than other breeds I have worked with or owned but think that has much to do with their breeding and handling. When the time is right you will find the horse that suits you. Right now your experience is with gentle willing horses of all shapes and sizes at a therapy place. In the end you are looking at personality and a horse that is capable of doing whatever you are most interested in. It doesn't necessarily limit your breed choice but if you choose to compete it may limit the level you compete at. At 5' I would look more for comfort level mounting and dismounting in various situations and not so much about your weight unless you are grossly overweight and from pics that doesn't seem to be the case. If you ride well and are balanced your weight on a horse isn't going to be something you need to focus on.