Sigma is only putting the EX designation on lenses that have a constant aperture. The 150-500 is an EX lens in every sense of the word, just without having EX on the lens.
It is a better lens than the 50-500. The only reason to get the 50-500 over the 150-500 is if you need the 50mm.
tnx! will be on my shopping list! great for the tracks I'm sure... but the 50 - 500 doesn't have a constant aperture then why the EX?
Edit just read the ex means..
The exterior of this lens is EX-finished to denote the superior build and optical quality, and to enhance its appearance.
so i guess its just the finish?