I write in 3rd and 1st primarily, and every once in a while I'll play with others. I'm well into a novel, a little over 40k words, and it seems to me that my dialogue is just on, it's sharper, more meaningful. It also occurred to me that it might have something to do with writing in first person. My last two books have been in 3rd.
There is the notion that it's easier to get into a character in 1st, easier to play the role, easier to feel what the character's feeling, easier to find the voice, but does this have a positive influence on the dialogue you write?