UPSCALING DOES NOT ADD QUALITY - IT REMOVES DETAIL.
Now that I've got that little rant out the way.... the point of upscaling is to be providing your video signal with a *better* upscaler than that which you have been currently using. e.g. TV has a pants upscaler, so use an AVR with a better one built in and upscale your signals this way. Upscaling is a necessary evil one way or another otherwise you would have a 720x576 sized image inside your nice HD Ready display!! In the case of the Pioneer I can't think of a flat screen with better upscaling, the Onkyo could very well be a minor step backwards! The Onkyo also does not have very good film detection, whereas the Pioneer Pure Cinema modes are excellent at film detection. A more significant backwards step. Using s-video connection rather than RGB scart throws away half of the colour resolution of the output straight away just be downconverting to s-video, so again a backwards step. Short story long I would stick with what you have.
Oh also with TVs which are 1024x768 or 1360/1366x768 you have to consider what you are doing when you use something to upscale SD to 720p. Upscaling is a lossy process, so you upscale 576 to 720 lines and effectively smooth out some of the detail. The TV receives *already smoothed out* information, and smooths it out that little bit more to get it to 768. The result is a picture which isn't exactly detail rich. A general rule of thumb is the fewer upscaling processes the better. This is a hard rule to follow mind since sometimes in weirdo circumstances you are gaining more than upscaling (e.g. noise reduction or better connection type).