The 86 is very raw and 'race' focused, whereas the Z4 is more refined - perhaps a little more grand tourer than the 86, if that makes sense. The engine on the 86 has to be worked very hard since it is normally aspirated and that could get somewhat wearing on occasions. I agree with your thoughts about the comfort and certainly the build quality is nowhere near as good as the Z4. That having been said, the seating position in the 86 is well nigh perfect.
My 86 was, I believe the first in Essex and so was nearing four years old - time for a change then and after looking around decided the Z4 was the choice; not very scientific, but I'm very pleased so far. I decided on the 20i, which is faster than the 86, but I tend to use my bike for a fast burn and really that knocks most cars into the background!
To conclude on the 86 though...great fun, very practical and I thoroughly enjoyed owning it - hopefully will be able to say the same about the Z4 in three years time.