I would definitely say SSAT is much easier. I don't have a choice of taking the SSAT; I have to take the PSAT or SAT. On the SSAT, I could easily be in the 90th percentile, while on the PSAT, that would be a bit of a struggle.
Schools would also prefer the SSAT to the PSAT, unless applying to 12th grade or PG. I feel that the majority of applicants send in SSAT, so it would be easier for them to compare scores.
Also, the PSAT is taken by almost EVERYONE in 10th or 12th grade, while the SSAT is taken by a much smaller group. So chances on getting a higher percentage on the SSAT is more likely than on the PSAT.
In general, the SSAT is FAR easier than the PSAT.
You may be wondering about the ISEE, however. I have also taken that, and I still find the SSAT to be SO MUCH easier than the ISEE or the PSAT. Also, most boarding schools would prefer SSAT over ISEE.
But there's no harm in taking them all, and then sending in the very best!