DD has taken both.
Her SAT scores are not that awesome (well, okay-- for anyone else they might be, but frankly, she had a really bad day and it just isn't a test format that favors her).
The math section on the SAT is very... er... peculiar. It's pretty coachable, IMO, but it's also got some quirky stuff that isn't a realistic assessment of math skills, IMO.
The sections on the SAT are much shorter-- most about 25 minutes. It was far less about reading speed and time pressure, but DD calls it the "ADD version of a standardized test" because of all of that hopping around.
It also means that a speedy test taker will spend MOST of that 5 hours hurrying up and then waiting.
The math and writing sections were the only places where DD had any time pressure to speak of on the SAT.
ACT-- well, we don't have her scores yet, but we expect her math score to very likely go from 90th percentile to 98th, and for everything else to climb to (or stay at) 99th.
She found the ACT much easier, even though the math section on it is technically more demanding. It's much more about conceptual understanding, not single-method "trick" questions like on the SAT. In other words, it didn't matter if she knew the "right" way to work a problem as long as she knew A valid way of doing so, if that makes sense.
Writing sections-- this is a 30 minutes-to-longhand-write-an-essay thing. Same on both tests, but it comes FIRST in the SAT and last with the ACT. You can still take the ACT without that section. For younger kids who aren't yet ready for the writing section, that might sway me right there.
Science section is only on the ACT-- and it requires a LOT of reading and processing speed.
From what I can tell, while both tests include time as a pressor, the ACT is far more extreme in this respect. The sections are longer, true-- but they get 60 math questions to complete in as many minutes. All in one shot.
DD was one of only two people in her exam room who were easily finishing all of the sections with time to spare.
For example, that science section involves reading about experimental design or results, and then answering insight-related questions about it. 40 questions, 35 minutes.
My personal recommendation is to do a full length practice test for each one (check a local library for official test-prep books) and then decide which format and style suits you better.
