The main difference between the two versions of puppy lie in the repos attached to them. The packages on offer with Precise are a bit better than Slacko, with a few more available to download. Even with the higher number, as Precise is based off of the latest Ubuntu LTS, these packages will have some backports and generally be stable with better maintained versions. The Puppy developers themselves believe they will make this version of 5.4 long term service as well, although to be honest the compatibility with the packages from Slackware is still noticeably better than with Ubuntu counterparts. You need these extra repos anyway, as the solely Puppy repos have little more than extra drivers.