Unless you're using a fairly outdated machine, or are using some older tools that haven't been updated to work in DS4.6, I wouldn't be using DS3. DS4.6 will handle all of your older content (except for certain older tools, as I've already mentioned), and I personally find it much easier to use, customize, and organize than DS3 or even the newer versions of Poser. (Actually, I don't user Poser at all anymore: that outdated interface irks the hell out of me.)

DS3 is also not being updated anymore, while DS4 is (which is kind of a mixed blessing -- some updates are better than others, and some seem to add new issues -- the current version of DAZ Studio Pro 4.6 (4.6.3.52) seems to be a bit more touchy than the previous version).