Update:
I committed a test following a comment and four upvotes to very comment. It turns out that what I had said is the correct answer. Here is the result:
http://jsperf.com/hasclass-vs-is-so
The is is multi-purpose, you can for example do is('.class'), is(':checked'), etc which means is has more to do where is hasClass is limited which only checks for a class being set or not.
Hence, hasClass should be faster if performance at any level is your priority.