Given the choice, I would almost always give a little change, because I can remember times where I needed to use a payphone and only needed 50 cents, and it would pain me to leave someone else in that helpless position. I do say "almost," however, because if the person in question is quite obviously a drug addict or a drinker (and one can usually tell), I usually say no or lie and say I have no change to spare (although nowadays I don't usually have to lie).
I've never come across a beggar who's insulted me, though. In fact, I remember in Chicago where I met a man who was old and obviously homeless; he asked me for money to grab a bus ride, and though I was initially apprehensive, he proved to be probably one of the nicest and most insightful people I've ever met. We had a conversation about relationships, women, and life in general, and then when the bus came, he bade me farewell and blessing and was on his way to who-the-hell-knows-where. To this day I don't regret one bit handing over the money. For all I know, my generosity might have changed his life.