OK my two penneth....
The driving experience on all of the four variants of the XK (Coupe / Convertible 8/R) are different - the R's have a harder ride, so not so good for roads with an uneven pavement because of their CATS suspension and lower profile tyres... The convertibles some say are subject to scuttle shake... What's the neighbourhood like for a soft roof? I have heard that with the convertible, the rain runs straight off it through an open door (It has no gutter)... Also I guess you are in the US, where abouts what is the weather like - is it ideal convertible climate - coupes are possibly better for the desserts and wetter northern climes...
Basically talk with your dealing - borrow SOME XKs compare the ride quality of the 8 vs the R. Also try the convertible, get used to the hood arrangement, can you cope with the cover needing to be clipped in place each time you lower it.
Don't let your dealer fob you off with an 30 minute/hour test drive have each one for a couple of days. Try your regular driving pattern in it.
Thing to remember is its a big investment on your part. And if you get it right, one you will love. I have had a XK8 Coupe for 2 years and am still enjoying my "honeymoon" period.
Hope this helped
S