I have Debian 8 and Debian 9 here at the moment and waiting for the BunsenLabs Helium (Debian 9) ISO to come out. I like Debian because of it's ±3 year cycle and will probably keep D8 running as long as I get security updates IF this old computer lasts that long.
Installing apt-listbugs in Debian SID and being careful about holding back critical bugs, SID is really quite stable as well. Two things to watch for on updates:
1. apt wants to install a new kernel and you see 100+ programs to be removed: STOP do not upgrade.
2. 'python' - when you see problems with python ... back out and wait a few days
They are pretty quick to fix bugs in SID - more so than in testing IMH experience.