I, as a rule, do not give money directly to beggars. I'd much rather take them to a fast food place and get them a meal than give them money which they may spend on alcohol or cigarettes.
I think you would have to define "disabled". There's a large percentage of the homeless who are mentally disabled. I would probably simply give them a few dollars because, since most of them are statistically paranoid schizophrenic, they would probably not accept any help as far as buying them food.
Someone who is physically disabled, however, I would much rather help through a charity. Physically disabled people have the exact same opportunities that everyone else has in my country, but they may be down on their luck financially, which is where charity comes in. Simply giving them, or any beggar, money is simply keeping them from finding their full potential in taking initiative to improve their lives.