What you get for your money, Onkyo has been pretty tough to beat for many years. I'm definitely a fan of their equipment. Pioneer also makes some very nice high end equipment without a doubt. Don't believe you could go wrong with either one of them to be honest.
Main diffferences:
Onkyo has more HDMI connections, 6 vs 4, although I don't really see the use for 6 HDMI inputs. I mean you have what - computer, satellite or cable, Blu-Ray, HD-DVD, XBox 360 and PS3. Don't know many that would have more than any 4 of those connecting to a single TV.
Pioneer has bested Onkyo in:
1080p upscaling vs 1080i (of course without them sitting side by side, you never know which has the better upscaling) and
Built-in USB for your iPhone/iPod with onscreen display vs having to purchase a separate add-on
Onkyo does have 2 yr warranty vs 1 yr on the Pioneer. But with these two companies, highly unlikely you'll ever have a warranty issue. Both these companies build equipment to last.
Very tough choice, if I were looking right now, believe I'd go with the Pioneer this time. Don't think I'd have anything negative to say to those that decide in favor of the Onkyo.
Tough choice, can't make a wrong decision here, just the one that best suits your needs.
Check out the home page for each if you haven't already.
Onkyo TX-SR607 - 7.2-Channel Home Theater Receiver | Model Information | Onkyo USA Home Theater Products
Pioneer USA - Pioneer A/V Receivers
I have the Onkyo 705.