I understand that some packages (like PSTricks) can't be used with pdflatex. Are there any other differences between these tools?
Can we say that pdflatex is better if we don't care about ps?
Some packages only work with direct PDF output.
For example, there’s PGF/TikZ which is analogous to PSTricks but much more powerful in some regards.
There’s also the microtype package which improves LaTeX’ line-breaking algorithm by using microtypographic features of the font. Let me illustrate (left: LaTeX, right: pdfLaTeX with microtype):
[Taken from the TeXblog]
And of course there are the packages that use special PDF features, as mentioned by Dima.Tweet