I took both (since I could just use whichever I scored better on for applications), and the ACT is a much less miserable format. The test is broken up into much larger time chunks, meaning I could take a more useful nap after finishing a 50 minute section in 20 minutes than I could finishing a 20 minute section in 8.

Difficulty wise, the ACT was easier, but not by a lot. They seemed to have pretty similar predictive strengths, based on percentiles of people I knew lining up between the two, but obviously the ACT loses precision because of how coarsely it's scored.