I believe the Onkyo is higher in the Onkyo line than the Pioneer is in the Pioneer line. However, some people online have claimed to have reliability problems with that Onkyo, so that is something else to consider.
If the Pioneer does everything you want a receiver to do, and it is already hooked up, I see no reason to change which one you are using. I would probably sell whichever one would get me more money in that case, if I could find a buyer. That probably means selling the Onkyo.
On the other hand, if there is something you would like the receiver to do that your Pioneer does not do and the Onkyo does (e.g., 7.1 sound, etc.), then I would switch them out and sell the Pioneer if I could find a buyer.