I've liked Marantz in the past - but the AV7702 doesn't have HDCP 2.2, so if you do plan to do 4K be aware that most likely the content - unless it's streamed directly to your set - will likely require HDCP 2.2 (think UHD blue ray players). If you're going to stay squarely in 1080p and lower until your next pre-pro, then the Marantz would be fine. The Marantz's in the past have tended to sound a bit more "musical" and less "hi-fi" than other mass market brands - but I think that's changing over time and they are beginning to sound more like their sister company Denon. Onkyo (and sister co. Integra) on the other hand veer more in the opposite direction - which might be ok for action movies and gaming, but could get more fatiguing for concert videos. The amp and cabling you plan to pare it with may also influence your decision. Ie if your amp + cabling veer more towards the dark (eg Theta + Audioquest), then the Onkyo may be best. If the other (say Krell / Bryston + Nordost / some Kimber), then Marantz may be better.
Lastly I've found the room correction software from Audyseey that Marantz uses usually to give good results. The Pioneer MCACC (?) also seems to work well. This software is pretty important for less than ideal rooms when your running so many speakers + sub(s). I can't comment on how good Onkyo's equivalent, AccuEQ Room Calibration, works as I don't have first hand experience.
Hope that helps!