Right now, Sangakkara is the more accomplished batsman and just looks more classical than KP but I reckon, once KP really puts it together, he'll be a world-beater. Has all the tools; hits the ball as hard as anyone, shots to all parts of the ground (doesn't seem to have as obvious a leg-side bias as he did early on), plays pace and spin very well, rock-solid defence and a real ticker for the fight.
My impression of him is formed right from his first Test; in the midst of one of the worst Test collapses I've ever seen and McGrath bowling brilliantly, he was almost serene. If you didn't know, there's no way you'd have thought it was his first Test. Some batsmen never play that well under pressure. He was even more the complete player on his Ashes 06/07 tour, his ton in Adelaide was a gem. One of those guys who is constantly re-assessing where he wants to be, how and why as evidenced by his transformation from a bloke who hit a long-ball to the more patient player we see today. Personally, I think he's gone too far into the patient territory and once he strikes that balance, he'll be awfully tough to stop in any conditions against anyone.