The SAT is a far more predictable test than is the ACT. That being said, the SAT is brand-new and not all the kinks have been ironed out yet. There are only 6 official practice SATs thus far, but there are over 50 real ACTs floating around that you can practice with.
All things being equal, I think it’s now easier to get a higher score on the SAT (until the arrival of the New SAT and its inflated scores/percentiles, it was the reverse scenario), especially given the ACT’s very liberal stance on extended time administrations of the tests.