After enrolling in FB you will be FB Ivory. Only after obtaining the minimum number of segments / level miles (EQM) will you obtain FB Silver.
Please note that it may not be beneficial to switch to FB only for hotel bookings as you may earn less miles in FB than in SM (specially if you buy discounted economy tickets).
There are a few threads on this topic in the FB forum which should answer your questions what is the better programme (FB vs. SM).