I had the RP 6 and RP 8 at home. I'd take the 6" but the 8" was very nice too.
tough call... I remember thinking the 8" was too big for my room back then.
IMO this is your personal preference, and where music is a subtle determining factor.
If it is Techno Rap with heavy LowEnd I'd be getting a 8"....if its more guitar and vocals and harmonys etc,,...probably a 6".
the gang here lead me to the 6" when I got my first ones. Physical size and Physics the 3ft triangle....6" was an average....but as you can tell, 5-6-8" is the range for most nearfileds.
However, after recently running some 8" here at the house, one can remove bass with the low cut switches easier than a person can add bass to the 6" should it be required. So I'd have to give the 8" a "Versatility" edge for that reason, in that a 8" can remove the sub and more easily match a 6" upper mid range ability.
good luck...