I finally bit the bullet and get a permanent PSR-800 and after years with the PRO-106 I can say a few things. If your scanning is pretty "locked down" in terms of knowing exactly what is out there, and how you want it organized, and you aren't doing a lot of searching, the 800 has my nod.
First off, and this is a huge thing, the audio on the 800 is far better, and louder. For those who use their scanner in a car, it matters big time.
Secondly, the display is better (though this is WITH a back light which is pretty much mandantory), and shows more information on RID, and things like when a TG is patched. You can control what happens with the display more too.
Third, the scanlist / scanset combination gives more flexibility of scanning combinations. You can turn off not just talk groups within systems, but systems within scanlists, and scanlists within scan groups. For the more advanced user in a large metro area, this is important.
Once you learn where the buttons are by memory, things like pausing and resuming, skipping, etc., are easier to do on the fly, in the dark on the 800. And this is an important practical point for every day scanner use.
Aesthetically, it doesn't look like something from the 90's as the 106/500 does. Not a big deal to some I suppose, but I was tired of the outdated clunky look of the old scanners and welcome a more modern looking unit.
The power / data connection is FINALLY USB, a common and now nearly universal interface. A "data cable" can be had on ebay for a buck, not $30 dollars as a 500/106 cable cost, and a car charger is just a mini usb charger that many phones in recent years have used. Anything that supplies USB power supplies power. Having an SD card for data mean you can have a backup card for under $10, and you can back the files up to your computer with eaze. As a side note, if you buy a larger card, but a CLASS 10 card as it really helps with boot / transfer times.
Recording on the device in a common file format is awesome, and not something the older units can do.
Having the ability to decode the new system types will be huge for those who get these systems, obviously.
The 500/106 do have advantages for advanced users that I miss and they are worth noting. You can control them with a computer and stream them over the internet.You can control how the signal bars relate to actual signal strength. You can tune a given frequency directly and quickly. The display was good without a back, light, party contributing to the fact that these older units are better on batteries. Digital decoding is a bit better on these older units in fringe areas, though by playing with thresholds on the 800 and ditching the provided stubby antenna and installing a standard length model, this issue can be minimized greatly. The squelch circuit is a bit better on these radios.