Dunlop all the way for me as well. The Schallers had a tendency to flip upside down on the strap, and although they still seemed to stay firmly attached I never quite trusted them. The locking mechanism was also a bit more fiddly.
I prefer the Dunlop buttons that also double as regular strap buttons, since I don't use strap locks at home (usually), but the ones that sit flush with the instrument are definitely pretty appealing in terms of looks.
Before I started using "real" strap locks I used those Dunlop plastic ones, and they worked really well. Really secure, cheap and easy to use.