Hi, you didn't mention what amp you own currently or perhaps this is an all new system? The reason I ask is it's good to know what you're familiar with in terms of features, sound quality and usage. Looking at your options though and your comment about the room and it's size, I'd consider the Pioneer not worth it. The Pioneer could adequately fill a large room well, but since yours is a medium size with acoustical issues, I'd select the Denon with its superior room correction system, Audyssey. I've owned 4 Denons over the years(and 3 Onkyos) and you can't go wrong with Denon. The feature set is pretty good, and it'll drive your speakers well.
Something you might want to consider is ways to improve the acoustics of your room. There are threads on room treatments and techniques that might help you.