most the time, i pick a distro that the software i'm going to be using recommends...the one they develop on or have software install guides for. so, i start with the doc on the software page first to determine that. 99% of the time, its CentOS or Ubuntu for the things I use. i prefer CentOS and will look for software that uses that first.
as a full time OS X machine user, I keep VMware Fusion ready with base template of CentOS: one patched with current stable release and one patched major version behind release.