Well, source got better graphics, less people that play it. 1.6 got WAY more people playing it, really bad graphics, but WAY better gameplay... Personally, I play source cuz I can't buy online and didn't wanted to search in many shops for 1.6 ... but from my friend (that got 1.6) said to me, 1.6 is really better... harder, but way better. Source got too many new players, and as 1.6 is old, its full of pro people.
Well, I hope I helped you at least a little bit ^^