If the Sigma 50 had been smaller and lighter I'd have bought that instead. As it was I very nearly went for a new Sammy 50 f1.4 (Amazon had a deal £349 including dock) but the size/weight issue was the same plus it's not the best lens when it comes to focussing though still usable.
TBH I think that the Sony SEL50 f1.8 renders quite acceptably, but the focussing system is made of faeces.
That's the thing, it's usually weight v cost v af v aperture etc etc.
After using so many lenses across so many systems I've settled on arts, just a balance that suits me, the only penalty being weight.