As M3UOND said, the cost on the 850 is much higher. If you do get one, make sure is a '95 or higher. Look at the total production for these cars on one of the links. There IS less than 300 850csi in the country. These of course cost lots more.
The horse power is almost the same on both models, depending on the year. I have not heard any horror stories from other 850 owners but they do mention the cost for parts/labor. There is hardly any room to work on the engine bay.
These owners take pride in knowing there is a V12 under the hood.
Also, don't let prices fool you. Sometimes a good deal is not always the best. I do believe that these cars have reach their bottom. The lowest I 've seem them for is 10k with 150k miles or more... Remember, take your time, do your homework.
One more thing, if I had to buy one of those you listed, I would go for the 850. There is one thing that bothers me about that model though. I believe is call MDK or MKD motor, this sucker gets gummy and you have to replace it around 130-150k miles. Very expensive to replace and difficult to find. There is a forum call roadfly (E31 owners section) that are very knowledgeable on these cars. They can find or sell you parts. One guy, even bought another car so he could have spare parts!!
Last edited by LOS; 11-29-2006 at 10:10 AM.