Those two choices aren't even in the same ballpark. If you have the room, get the RTi8's for mains. The 6's IMO didn't sound as good as the 4's for surrounds (but I'd probably take the 6's as fronts over 4's for bass), but that's personal taste. Now I preferred the RTi10's over the RTi8's as they didn't have the bright highs. The RTi12's weren't an option as they didn't have floor models at the prices I found on the two smaller versions. Your room as well as receiver should allow tweaking of the mids/highs. I'm not sure if the Onkyo's offer internal eq's - but they are a godsend for balancing out room imperfections.
The 8's are crossed over at 2.2kHz, drop down to 40Hz, and handle up to 250 watts/channel. They are rated at 90dB. These most likely will sound more sibalant as the tweeter is handling more of the higher Hz spectrum.
The 6's are crossed over at 2.8kHz, drop down to 50Hz, and handle up to 125watts/channel. They are rated at 89dB. These may sound totally different based on the 6.5" driver handling more of the mid/highs.