If you want resale value to climb, the 911 will do that sooner (assuming you don't overpay to begin with).
However, cars like these make lousy investments, so I wouldn't stress that point in your search.
For a number of years I had a 911SC. I sold it last fall and began looking for a replacement.
I test drove a 924s, then a 944S, then an S2 then a 968. They all got progressively "better". I was close to pulling the trigger on a 968, only to have it bought out from under me.
Then I drove a Boxster. A 2.7l. Done deal. Fantastic car. For me, a car more modern than my SC was important. I wanted a Porsche that would keep me warm (without smelling), cool in the summer, have a top I could put down/remove and one that the wife could use without all the trappings of my Track prepped SC.
For the large part, a 911 is THE porsche to own. People won't run from you for driving another model, but to many, a 911=porsche and all others are not desired. Sad thing is, even the most revered "aircooled" 911 is now 9-10 years old and getting older. They are robust. They drive and feel like no other car.
Having a 924S currently, a 968 might be a great progession. You will be very familiar with the car and its vices and plusses. A Boxster or 911 will require a learning curve (which can also be fun).
The 986 has not stopped depreciating. Porsche has built 200,000 boxsters. That makes them not so rare
My 911 ran fine. The money put into it was oil changes (10-12 qts of mobil 1) brake pads and fluid and tires. Prices for these items are similar for boxster and 911.
Spend some time test driving samples of all 3 models. Only you know how the car will be used by you.