New SAT vs ACT

<p>Ok! I am fairly new to this...Now beginning process with D2. What are the major differences between SAT's and ACT's?
In terms of planning for success, when is the best timing? D2 is a junior.Dec, Feb, April? I know she is planning on taking SAT's in March.She worried about the new writing section, not her strength. She suffers from test anxiety which could cause some high drama.
Do most schools accepts EITHER/OR, consider BOTH?
What is the best way to prep this kid?
She is a good student,A/B, carrying 4 AP classes, APstats,APcomp,AP UShist,AP Spanish,pre calc,physics.She plays with youth symphony x 3 years, also university wind ensemble,all state, all region in her instrument,runs varsity x-country & track x 3 years
Thanks for any info</p>