I prefer talk, then roll, and what the player says and how persuasive he is in real life is less important than the personality displayed by his character.
(NOTE: I posed the following in another thread, but it also seems relevant for this one).
I would allow role-playing to affect the outcome of a skill check, but ideally, I would not allow player knowledge or player ability to substitute for character knowledge or character ability.
Let me explain.
At its most basic, D&D can be played like a tactical wargame. Make a Spot check, roll initiative, roll to hit, roll for damage, cast magic missile, make a Reflex save, make a Tumble check, make a Bluff check, make a Heal check, etc. Player ability and player knowledge are completely divorced from character ability and character knowledge. Even if I'm a black belt in karate, my 2nd-level sorcerer doesn't have any special unarmed combat ability unless I give him the relevant feats or monk levels. Even if I'm a doctor or a certified first-aider, I can't describe to the DM the actions that my character is taking and expect to stabilize a dying comrade without having to make a Heal check using my character's Heal ranks (if any) and appropriate modifiers. I shouldn't even be given a circumstance bonus!
What elevates D&D from a tactical game to a role-playing game is that my character is not just a collection of ability scores, skills, feats, class abilities and equipment. He's got a personality, which is something that cannot be defined by game statistics.
So, how would I allow role-playing to affect the outcome of a social skill check? Well, if the PC displays characteristics that the NPC admires, I would give a small bonus to the skill check to represent the fact that the NPC might be more favorably disposed. For example, a PC who shows that he is willing to brave great dangers to rescue a comrade would get a bonus to Diplomacy checks with NPCs who admire courage, determination, or loyalty. But he still has to make that check, and if he has a low Charisma and no ranks in Diplomacy, he still isn't going to get much help.