FF designates flow formed production as opposed to low pressure cast, so they'll be lighter and stronger. If it's within your budget, spring for the FFs.
Generally speaking:
In strength (and price): low pressure cast<flow formed<forged
In weight: forged<flow formed<low pressure cast
There are some other manufacturing methods and specific wheels that challenge that generality, but it's a pretty good guide.
If you want details on even more manufacturing subcategories, check here:
http://www.tirerack.com/wheels/tech/techpage.jsp?techid=90