I went with an Ultradot 25 on my MK II slabside, and am very happy with it. It's not too heavy, no halo around the dot, and it goes for a good price. And the bonus is that it gives the gun that nice Han Solo look.
Just put the Ultradot 25 on my 686 revolver. Unbelievable! First time I ever shot a 1" group at 25 yards. At 15 yards I can stand and watch the holes appear... groups the size of a quarter. in bullseye competion (15 yard stage), I actually select the part of the 10 ring I want to rip away so I don't waste ammo shooting through the center hole! You won't believe how well you can aim with this thing.
After much research, I got the UltraDot 4 to put on my Ruger MKII. Check out Larry's Guns It gives you 4 dot sizes to choose from. I got mine on sale for $165. I checked their site today and they are still on sale.