Stephen Briggs, all the way.
Disclaimer: I've only heard Briggs and Planer.
Briggs just has a much better range of accents and a deeper reading of the characters, whereas Planer seems to aiming for humor, rather than personality.
Planer's Colon sounds younger than Nobby (in Jingo). Brigg's Colon sounds older, more experienced, and appropriately, stupidly pompous.
Planer's Carrot sounds like an innocent kid. Brigg's Carrot sounds not only like the king Carrot is, but of a dwarfish king.
Planer's Death sounds like a befuffled aristocrat from a P.G. Wodehouse novel. Briggs's sounds like--well, if I had to be visited by Death, I'd hope he sounded like Briggs.
Planer's Lady Sybil sounds a very large woman. Briggs adds gravitas to her gravity.
Planer's Vetinari sounds like a country squire. Briggs's sounds like a mildly benevolent yet always menacing tyrant.
Jeff in Boston