There are about
a million different brands of mesalamine drugs (and I've taken like half of them honestly) and mostly they are interchangeable but some are designed to target different areas of the GI tract and may have some other differences.
As far as I know Pentasa releases pretty high up in the small intestine, and it sounds like Salofalk releases lower in the small instestine
so if I'm not mistaken Pentasa would be more effective for Crohn's and Salofalk might be more effective on UC (since more of the drug presumably reaches the colon).
Salofalk and Pentasa also have 500mg size, I have some of both right now. I've used both for maintenance of UC remission and didn't notice any difference.