the difference between Macro lenses

Hi.

I know most people in these forums favour Pentax lenses but I wanted to see if anyone had an feedback on a couple of the Sigma Macro lenses.

In particular I'm interested to know what the practical differences are between the 180mm F3.5 Ex and the 150mm 2.8 Ex?

Obviously there are things like size and weight but in terms of the operation is the only major difference the minimum focus distance (the 180mm lets you be a bit further away)?

Any feedback would be great

Thanks

J