Some require adding a tall front sight, with cross pins, ruining the slide and not fitting the holster anymore...Millet comes to mind, for one.
There are some that are low enough to use the front sight as is, but hang off the back of the slide, over the hammer.
I opted for the pricey adjustable factory rear sight, with a windage adjustment (two opposing screws) and 3 blades, each with white dots, for elevation...it looks original and fits the holster...$72 + shipping.
Other than price, I have no regrets.