crudely speaking look at Desktop environments and windowmanagers first
speed of updates second
security depends on whether you run home or server services
gnome..ubuntu....Kubuntu if you like that ubuntu style with kde...tends to lag behind gnome
kde ...mandriva offers both but kde preferred
kde...opensuse has both..its configs are slooooow
with 512 Megs both kde and gnome will be slower..kde prolly slower than gnome.
of course I choose sidux for kde, fast updates based on sid...security is handled by unspeakable applications....what are they....well I can't say ....they are....unspeakable