I just signed with BMI about 3 weeks ago. I have yet to actually register a work, but I have some things that will be coming out towards the end of the year so I went ahead and registered. Many of my friends that have work out there highly recommended BMI as they are already signed with them.
In my experience, I saw them to be a little more personable as I have personally interacted with several of their employees in business and social settings. I really weighed the two out, and just kept coming back to BMI. The main thing with those companies is that they track the plays of your work and make sure you're paid contingent upon you properly reporting and registering your work upon completion. As for those testimonials, I wouldn't give them much weight. Some people just like to stir up trouble on the net.
Not to throw a monkey wrench into an already difficult decision, but have you looked into SESAC?
As or songregistration.com. if they ask you to pay some kind of fee, then they just want your money, and you may actually be signing away the rights to your songs. Most people don't take the time to read all of that small print and fall for the okey doke. I'd walk away from them. As for copyrighting your work, you can do a "poor man's" copyright by mailing a sealed copy to yourself, get a postmark on it, and keeping it sealed and it can be just as good as going through paying some fly-by-night company.