I love my Sigma 28/1.8, which is sharper and equals the 50 in terms of IQ, but the 50 is more versatile.
I love my Sigma 28mm too, such an underrated lens but I find the 50mm FL much easier to use as a do-all lens. I use the Nikon 50mm 1.8D. Have been thinking about upgrading to the Sigma 1.4 though.