[monoflo]

Both are interesting cars --21K for the 86 sounds good given mileage and quality. Boxsters will take a long time to hold value.

Driving experience will be different --as already said the Boxster will be easier to drive well --the 911 more rewarding once mastered. The 86 shifter is not a modern device --the boxsters is.
I have an 85 so I know of which I speak.

I motocrossed the boxster and mid engine is one way to get fine handling. Much better ergonomics if that matters to you. Again an easy car to drive fast.

911s are iconic --never will be the "low end Porsche" and many consider the 84 -89s the last of the old 911s. This does not make the Boxster a bad car just it does not have the history. Heck I love 928s.

So I guess I lean to the older car given your low mileage projection --- you can probably get your money out at the end and get that Boxster S at that point --No real bad choice either way --Have fun