I think you answered your own question. If you love your current Boxster there is no reason to ditch it for a used one you now nothing about (maintenance history, etc.).
Personally, I prefer the styling of the 986 over the 987. The 986 was designed to be like the 550 Spyder, the 987 strays from that design so it can be new. Whether or not it is better is up to you.
The only significant change worth the $30k upgrade cost in my opinion are the built-in cupholders.
