<2 cents> I would recommend Slackware. Why? Slackware is the distribution which tries to stick to the basics....and that means an "uncomplicated system", which is what you want when installing and testing alot of the "bleeding edge software". The distributions that use rpms and various package managers introduce an "installation-and upgrade-simplicity" at the price of complicated package interdependencies, which come back to bite you try to install bleedind-edge software from sources. On the downside, Slackware does require a bit of linux knowledge to get it going. It is cheap too.

If you want a bigger, pricier distro, with alot of support, automatic security updates, great support thru maillists, etc. try SuSE.</2 cents>