DD took both-- SAT at 13, and ACT just after turning 14, six months later. Because of her acceleration, she did this between junior and senior years in high school, however.
Her scores on them both were (overall) 99th percentile, but she found the format of the ACT much MUCH easier.
The SAT? It was too much down time, too often. It really put her off her game to have it so choppy-- the short sections were awful.
One thing worth noting is that her practice scores for the ACT were about what they were for the real thing, and with the SAT, her practice scores were at the ceilings, and the real thing was 50-60 points below practice scores. In the case of the math sections, that meant a sub-700 score for her, which is mind-boggling given what I know of her math ability.
ACT, much better reflection of her abilities.
