I agree and disagree with different points of the other answer.
ABEC ratings are not the way you should rate it as, but for whatever reason, that's what a lot of companies do. For those that do refer to ABEC ratings, the higher the number, 1-3-5-7-9's, the better. So to your question, 7 is better. The important thing like the other guy said, is the company. If you are picking between 3's and 7's, i'd easily go with the 7's. BUT if you are picking between two sets that are both ABEC 7 or one is 5 and one is 7, then I'd look into which company is better and reliable.
I have ABEC 5's in my rollerblades.