"What's the most you ever lost in a coin toss?"
On a serious note, the Onkyo has power. It's a 50 pound monster receiver, it can dish out power like no other in it's price range. But it comes at a cost none the less. The TX-SR805 runs hot enough to cook most meals on, so it needs to be well ventilated. Also consider that heat and electronics don't get along, and that it may shorten the life of the receiver and other electronics nearby that may absorb some of the heat it passes.
Both are great receivers that can power most any speaker arrangement. If you don't mind a bit of back breaking work (lugging the TX-SR805 into your setup) and have a serious upgrade bug (longevity may not be a major concern in this scenario), then the Onkyo might be right for you. If you plan on this being a lasting setup, look around a bit longer for different receivers that might fit your needs even better. If you find nothing that fits the bill maybe the Pioneer is the right receiver for you.