I feel there is a balance to be struck between a project that is popular (where if you run into problems, you will get good support), and one that technically gives longer-term support (but if things go wrong, that support might not be very good).

I haven't used a lot of different distros, but for me, Debian has been a good balance of those factors. You may need to do more upgrades per decade, but the ones that you do are more liable to go smoothly.

Just my 2¢ on the topic (: