Hi Zoltan, I also noticed that with Picstep and Mach2 (v6.11N) the jogging was not very smooth, with Mach3 it is very smooth. I only used Mach2 to test my steppers and Picstep boards so I don't know about toolpath errors, with my current test XY table and Mach3 I don't have any so far.
Another difference other than the version, is that I use Mach3 in a separate 'minimalist' XP instalation. Look for the optimalisation text file at Mach2 yahoo group.
Mach2 runs on my bloated 600 Mhz win2K at work, where I built the Picseps and Mach3 on my 1,47 Ghz laptop.