At any given time there are approximately 20-30 E34 M5s for sale, most on autotrader, a few on ebay, a few in other places such as the BMW CCA classifieds.
There are usually about 10-15 E28 M5s for sale in the same combined sources.
Total world production of the E34 was around 2300 IIRC, US production of the E28 was around 1200, with most people saying 1220 or so. (Don't believe 500, that was the original plan, but they brought more in and people sued BMW because of the ad campaign that mentioned 500 examples. Those folks got fat rebate checks for a new BMW.)
For those that like the E28 body style, but not the US bumpers, there are a few of the european, small-bumper E28 M5s in the country, about 3-5 maximum, that were gray-market imported in the 80s. A bumper conversion is possible, but extremely expensive.
I always wanted an E28 M5. It is what started my love of BMW, before they even existed. (I learned of the M635csi in 1984 and thought that engine would be great in the 5er, all at the tender age of 8).
I am hoping to pick one up as a third car in a year or two, if all goes well, but a boat and an apartment in Amsterdam may come first.
I prefer the dynamic qualities of the E34 M5. I have driven several examples of both, and the E34 is much more advanced and faster to boot. However, the E28 is just a classic and that alone may earn it a spot in my stable. Too bad I only have a 2-car garage. Tough life, I know.