On a scale of 1-10 in increasing difficulty, SAT would be a 6 and IITJEE would be an 8.
Though SAT is easier, it is still a more comprehensive test than JEE which focuses purely on science and mathematics. Furthermore, getting admission to a top university in the US requires a lot more than a high SAT score. Students also have to show their prowess in other vocations and secure LORs from teachers at their school, something the IITs don't bother with. It is for this reason, many question the diversity of the IITs at times.
In conclusion, SAT is easier than IIT-JEE, but getting admission to a top university is much more difficult.