Hi,
The hardest part on the SAT varies for everyone based on what your skills are. For some people it's math, for some people's it's not. My hardest part was the writing portion because I write fairly slowly and messily (meaning I had to slow down even MORE to make it legible) and it took a lot of practice to get my writing speed up.

The SAT is somewhat different from the ACT. There's no science or social studies portion and it's more skills-based. Rather than being asked to read several long passages, there is a combination of long and short passages, as well as more grammar-based questions. However, the writing portions are almost identical in terms of grading and what they expect you to do.

The other major difference is that the SAT, unlike the ACT, takes of a 1/4 for wrong answers on everything but the free response math portion.

For more information and sample questions, you can visit collegeboard.com.