TL;DR version of a very full email from Ian - they both try to do too many things too quickly without regard to the dangers/consequences of doing them without knowing what they and you (as the admin) are doing, but as long as you (as the admin) are aware of these dangers, plan ahead with a full exit strategy (BWPS can be a bitch to remove), and remember to take full backups of your install and database before installing any security plugins, I *personally* don't see a reason not to use one or the other (I prefer WF to BWPS, it's less "in your face" with it's security features, but as I mentioned, I look after both).