What is main difference between the sat-i and sat-ii?

When the old SAT became the new SAT I, it also changed its format, deleting one type of Verbal questions and adding a new kind of math problems among other details. The scoring formula was also changed. The SAT of today is considerably different than the SAT of several years ago.
Whether or not it can be judged as any better a test or any easier or harder depends on your ability to deal with standardized tests.

The SAT IIs used to be called the Achievement Tests. You would want to take an SAT II if you are going to be applying to some highly competitive colleges or universities that require three
SAT II scores. Usually, the three SAT IIs that are required are Writing, Math, and a science or language of your choice.

The purpose of the SAT II tests is to measure what you’ve learned over the year about a particular academic subject. The best time to take an SAT II is a the end of the school year, preferably the end of your junior year.