Reply To: Which OS?
There is a danger in getting too far behind the current OS.
New programs often are only tested with the current OS. If you get more then 3 or 4 versions behind, it can be harder upgrading.
On the other side, your existing programs may need to be upgraded before they will work with a newer OS.
There is a web site, http://roaringapps.com that has user compatibility reports.
However, going from 10.10 to 10.11 shouldn’t be that big a jump. And, it’s free anyway.
10.12 Sierra is coming out this fall, too.