This is going to be tricky as I'm only on my iPad (lappy is being repaired), which means much to-ing and fro-ing getsbrather complicated! I haven't yet compared a 75 to and 83, but I'll quickly run down the differences of the 83 and 73 (ihave that to hand), then I can see what differences there are between a 73 and 75.
One of the main differences is the remote control. The remote for the 83 is not only IR, but is also RF compatible, so the receiver can be placed in a cupboard, or in another room. This remote can be bought as an option for the 73 at a cost of £400. Given the difference in cost of a 73 and 83 was £400, it was worth it just for this feature, for those that could make use of it. The 73 used 24bit DACs, whereas the 83 used 32bit DACs, which was one of the major contributing factors to the improved sound quality of the 83 over the 73. The 83 was THX Ultra2 Plus, which was able to fill a larger room than the 73's THX Select2, even though on paper there was very little difference in their power rating. The 83 also added improved Jitter Reduction, Full Band Phase Control, and a component video output for Zone 2.
The 75 has been passed down many of the 83's extras, except for the RF remote, and it retains it's THX Select2 certification, albeit in Plus guise now. It now has Jitter Reduction Plus, the 32bit DACs, and Full Band Phase Control. It now sports AirPlay and AirJam, the lovely iControlAV2 app, will now stream FLAC, an extra HDMI output (7), and comes with a wireless LAN plug in just in case you can't trail a cable all the way back to your router. The major difference between the 73/83 and the 75 is the amplification. As you know, Pioneer are now using DirectFET instead of ICE for their digital amplification. This means that those who haven't heard Pioneer receivers can no longer comment on their tonal balance or how they will sound with any specific speaker package (without spending time with them). Having gone from an 83 to an 85 myself over the past few days, I'm just trying to make the time to get to grips with the 85 and put it through it's paces and see how it compares. I'll comment more when I'm in a position to
I will say that the 83 is built a bit better than the 85, even though their weights are almost identical, although this isn't going to affect sound quality. I think it'll be down to whether or not you want the features of the newer *5 series. Unless the RF remote and higher THX spec is Important to you, the 75 could well be the one to look at, although it will cost a little more than an 83.
I think you'd be more than happy with either once you've got it home, set it up, and settled down to a good movie, leaving you a smile from ear to ear once you hear it's clean, crisp, detailed accuracy.