Personally I'm not keen on the idea of clamping the leaf springs with the bracket arrangement that the SIFAB uses, I do like the SSL method with the plate under the mounting though.
I've been thinking about which is the better clamping method as I'm going to be fitting one soon.
The Sifab (and Mulfab) method of clamping directly onto the spring will in theory alter the movement of the spring but as it's right against the axle mounting I doubt it will have any noticeable effect. It's also well tried and tested over many years and fitments.
My slight worry with the SSL method (and remember I'm a great fan of the SSL front suspension system) is that it might induce some twisting moment into the axle clamping U bolts and I don't know what long term effect that will have on the U bolt tightness.
I think I'll go with the Sifab spring clamping system and see how the SSL method develops.