The EF 28 f/1.8 gets mixed reviews. I'd read Bryan's review on this site. He's certainly not in love with it.
Some people say that they have focus issues with the Sigma. I haven't and am very pleased, and I know that forum member Sean Setters is very happy with his.
I have no experience with the EF 35 f/2.
I sold my 50 f/1.4 for two reasons: it was often too long indoors, and the focus was unreliable wider than f/2.5 or so. It was maddening.
A good, fast prime is a nice addition to your kit.