I've never bought them or used them...but, I'll throw in my $0.02...
As an engineer, I've dealt a decent bit with magnets. For reef applications, I'd only buy the neodymium magnets. They're pretty much the most powerful for their size/cost. They don't come cheap though usually. We used them in the labs...and let me tell ya...they're pretty dangerous. Even tiny ones!!! I was working with two plates about 4" squared...and they could've crushed my hands more than likely. But, they'll hold big colonies through glass quite easily probably.