You will need:
1) good vocab - this is a MUST; if you dont have this, your reading score is capped at about 650 - yes, you WILL have to learn that word list in that prep book =p
2) fast worker (the time constraints are heavier than in act)
3) good essay writer
1) ability to stay focused (the questions are in general easier, so your guard might slip, and it IS longer, after all)
2) fast reading speed (for science section)
In my opinion, the ACT is easier, but play to your strengths and pick whichever test is easier for you. Also remember that the majority of east/west-coast schools ask for the SAT, and that the majority of midwest schools ask for the ACT. I'm not sure if it's an advantage to submit the one they ask for, or disadvantage to not - you may want to find this out.
1) Princeton Review prep book
2) old SATII writing book (for essay practice)
3) if u have the money to spare, bostontestprep.com
1) practice tests from 'the official act guide' by collegeboard
Sorry I don't have more advice about the ACT, i never really devoted myself to studying for it. Oh, wait. The science section on the ACT: You will either need an insanely high reading speed, a very fast comprehension of the main details, or both. It's easy to get bogged down because the questions seem more complex than they are, but in the end they all boil down to common sense. I remember the science section was the one i was really worried about, because having enough time to check your answers (all of them) is near impossible.
As a final piece of advice, whenever you even GUESS on a question on a practice exam, mark it off and consider it wrong. That way, you'll look at the answer later and fully understand it. Getting an extra problem right during a practice test is meaningless - what matters is do you fully understand it THIS time so you will always get the same type of problem right in the future.
And if you have a low writing score, don't sweat it too much. The MC section is actually rather easy after you learn the grammar rules in princeton review and apply them at bostentestprep.com. You'll find in the future that you will be far more concerned about your essay, which is rather unstable. (take it from me - 11 on the SAT essay and 8 on the ACT essay one week later =.=")