I recently bought the Sigma 50mm f2.8 ex having considered the other lenses you mention. (but discounted them fairly early as 50mm was the focal length I wanted)
The 70mm sigma seems to be the pick of the bunch in terms of sharpness but I wanted something for macro work that could double up for some portrait work as well which is why I chose the 50mm.
I haven't used the other two you mention but the 50mm get's extremeley close to produce 1:1 results so using it for insects (other then dead or very slow ones) it's pretty useless. (I knew this before I bought it)
That said the pictures so far from this have been excellent with the only failings being my own.
One thing I was interested in was that the Sigma would cover full frame as well (the pentax 50mm macro did not) so it may be worth checking this with your choices so that when Pentax/ Samsung hopfefully do offer a full frame body your lens can alossbe used on that should you choose to.