The biggest difference between the two is that the Onkyo 805 will decode Dolby Digital HD, DTS HD Master and uncompressed PCM. The 805 also has digital room correction eq wich many believe is a huge improvement in sound especially for mid-fi.
Marantz has a better reputation for sound quality mainly due to their reference series. I would bet the difference in sound would would quite small in their mid-fi gear. I am not sure about the 8001 but the 8002 does have an oversized power supply wich could be benificial for hard to drive speakers or while driving 5 to 7 speakers at high levels.
I would get the Onkyo with an external amp.
Marantz has a better reputation for sound quality mainly due to their reference series. I would bet the difference in sound would would quite small in their mid-fi gear. I am not sure about the 8001 but the 8002 does have an oversized power supply wich could be benificial for hard to drive speakers or while driving 5 to 7 speakers at high levels.
I would get the Onkyo with an external amp.