I don't think the added weight will be a big deal.
That steel pistol is substantial already... a few more ounces shouldn't be a deal breaker, especially since the weight will be more or less directly above the shooter's hand.
I like the UltraDot tube sights. I've owned three (still have two of them, the first was sold for college tuition money in the early 1990s). The battery life is incredible, but always try to have a spare battery in your range bag/box.
I've never used a reflex style sight, but I'd sure like to try one. They look very "clean" to my eye. Trying one is definitely on the "to do" list.
Reflex sight offerings from Burris and Vortex seem to enjoy a good reputation, at a fairly reasonable price. Do some research, as the models have undergone some evolution. For instance, some older models (of at least one of those two makers) needed to be removed from the gun in order to change the battery...
Based on my experience with the UltraDot tube sights, I have to believe that the UltraDot L/T reflex sight would also be a quality choice.
I think some time holding sight-equipped pistols (the closer the gun is to your MKIII, the better) in a gun shop, or better yet, shooting at a range, should give you BOTH the answer that you seek...
I know that at my club, members are often willing (even eager!) to have you try their toys. If you're not a member of such a club, maybe a commercial range nearby might have a rental gun w/a sight that you could try.
Good Luck, and Happy Shopping!
