You may hear students assert that one test is indisputably easier than the other, but again, it really depends.  For almost all students, preparation will be required for both.  Neither of them tests intelligence; both require critical thinking skills which can be developed and honed with practice.  In this way, preparation for one test will help prepare for the other. 

The Princeton Review does a good job of comparing and contrasting the objective facts of the tests.

Here is my subjective take on difficulty:

The ACT math section contains trigonometry and the SAT does not.  However, the trig on the ACT is very fundamental trig; the problems are similar to what you will find in the beginning chapters of a trig textbook.  The math sections are of comparable difficulty otherwise.  Edge: SAT

The ACT has a science section.  Sometimes this is beneficial and sometime it is not.  With practice, I believe any student can be successful at this section.  Essentially, you must interpret data and graphs and make conclusions.  This is the scientific method in test form and since many schools emphasize this in the curriculum, I feel that most students are already prepared for a section like this.  Edge: Depends.

Both the SAT and ACT have recently added an essay to the test.  The essay on the ACT is optional.  This does not mean that the colleges you will apply to will not require the essay.  But if you were applying to a school that did not require an essay, the ACT will be an appealing option.  Edge: ACT

The reading comprehension is easier on the ACT.  Almost everyone concurs with me on this.  The answers to the ACT questions are taken directly from the passage, sometimes word for word.  Read and understand.  The SAT will ask you to analyze and infer or argue against and for the author.  Edge: ACT

The SAT has sentence completion questions whose answers require some knowledge of relatively tough vocabulary.  The ACT may have vocabulary on it, but not nearly as much.  Edge: ACT

ACT grammar and SAT grammar test similar things.  ACT grammar tends to test some punctuation and sentence structure as well.  I think they are both of similar difficulty (or ease, depending on how you look at it).  Edge: Draw

Pacing on the ACT is tough; there is a time crunch.  The amount of time allocated per question is less on the ACT than it is on the SAT.  This may take some getting used to and if you are a slow reader or meticulous test taker this will definitely hurt you.  Edge: SAT

My scores on the SAT and ACT were comparable.  For me, the ACT required less preparation time and was “easier” in that sense.  I realize this will not be the case for everyone and whenever I am asked which test is “better” I give this same extensive explanation.