Will have a look at a Pioneer 52 at the weekend and report back.
I've never played with the 71 but I owned a 51. I still do in fact, though it's in a box awaiting installation at the folks house, if they ever finish building it and by which time I'll probably have told them they should get a streamer and a NAS.
In response to the OP. It really depends what you want it for. If you want a universal player, then it's a bit obvious really, you'd buy the Oppo. But I'd hang on for a bit of clarification as to whether there are differences between the 831 and the 83. It may be better to get the 83 if they 831 has had some of it's features removed, but lets see on that. The Pioneer 51 was a great player. But and it's a big but, it was unbelievably slow to use. Reliable, but slow. Slow to switch on, slow to open it's drawer and slow to load. But once running it worked perfectly well.
If you want to be ahead of the game then the piece of kit to take a look at is the Popcorn Hour C200 which is a streamer you can slot a BluRay PC drive into and also a hard drive. For the same money as the machines you've mentioned this is probably where we'll all be in about 3 years time, so you'd be an early adopter. It's what I'm considering doing and if I didn't already have the Oppo I might well consider a streamer. Especially if it'll stream my SACDs off a Network Attached Storage (NAS) device.
If you are serious about the Pioneer 71, then I think you'd do better to buy a much much cheaper by now, 51 and save your money. As I recall, the difference between the two amounted to a slightly different button format on the front of the player (real buttons on the 51 and a touch sensitive surface on the 71) and a slightly more advanced video chip in the 71 for use with component video connections only. So no difference over HDMI. They both stream everything via HDMI so they're basically the same machine if that's how you'll use it. I would have thought you could get a 51 for about £200 or thereabouts and if that's really the machine that would suit you and you don't need a universal player, then that might be the better bet.
Hope that helps.