It depends on your skillset. I took it so long ago that I don't even remember which test caters to which skillset, but I know that I found the ACT to be pretty easy. I remember that the science section was a test of your ability to read charts and think in logical progressions more than it was a test of actual science knowledge, and the grammar part was really easy as well.
As for which schools accept which, all schools will generally accept both, although a very select few will have a preferrence (I remember UC-Berkeley wanted me to take the SAT-II's IIRC as well). I got into schools from the northeast, midwest, west, and south through the ACT though.