I currently own a 1999 S-70 GLT after owning a 1994 850 GLE (non-turbo)-The 850 was sold in 99 after 160,000 kms. I have owned both cars since new.

My thoughts:

850
-feels more solid
-harsher ride
-looks a little more dated but the S-70 is not an exotic that will turn heads everywhere you go.

S-70
-much faster car (I am comparing a turbo to a non-turbo)
-I am able to use my winter tires and summer rims from my 850
-less problems and trips to dealer
-ride is smooth
-the lines on the car looks better
-interior is a little better thought out
-there are cupholders!
-the 'pass-through' is a dumb piece of plastic that has to come out as opposed to a 'flip down' on the 850-Why Volvo changed this is beyond me.

My understanding is the ac evaporator issue is the same for both cars-Touch wood, my S-70 has been fine (I did have to replace it on the 850).

My choice: Go for the S-70 if it is a 1999 or 2000 model. Avoid the 1998 since I have heard that there are issues with it. BUT if you can find a good 850 at a low price (avoid the 1993), there is no reason to avoid it.

Last but not least, you MUST get the TURBO (I have a low pressure one and it is great!).

DC