^ if you aren't planning on switching to ff, the 30mm is a good choice.
on the other hand, being limited to tighter shots by using the 50mm is a great way to expand your creativity. i've played with a sigma 50 1.4F and it's built like a tank.
with it though, you do get rid of any premise of "discreet prime" because it's about the same size as the sigma 10-20 (77mm filter thread) as opposed to canon's own 50 1.4F which is very compact
http://www.the-digital-picture.com/R...ns-Review.aspx
http://www.the-digital-picture.com/R...ns-Review.aspx