The weather sealed version of the DFA does not have an aperture ring while the 'plain' DFA does. This matters if planning to use the lens with basic extension tubes (no pressing need maybe as it goes to 1:1 on itself, I suppose) or film bodies. With the long focus throw a limiter would be nice, but, as pointed out, the quick shift feature compensates quite nicely in practice. The non-WR DFA has a a bit of an El-Cheapo / El-Rickety feeling to it, light and compact though (I understand the WR version of the same or the Sigma 105 would be the polar opposites in that department
If something happened to my non-WR DFA 100mm I suppose I'd get the Sigma 105mm for the focus limiter and the more solid build quality (this would seem to have a version of quick-shift too?) ... the 70mm Sigma offering seems like an useful focal length for portraits though ... hmm ...