Six is better than five and the GP100 is better than the awful SP101 (a gun that does just about everything wrong).
The larger frame GP100 is much nicer. You can get the speedloader into the charge holes...amazing! There's an actual rear sight. You even get a longer sight radius. The gun's heavy enough to actually use, though the SP101 is no lightweight. Neither gun fits in the average pocket, though it'll fit in overalls.