rcamuglia has the economical, effective "cure" just right. After a while you don't even notice the "dot" if you put it right in the focus point of the lense. If you forget it some time, just use the sticky part of a Band-Aid cut to size, or anything that is sticky.
With regard to the choice of firearm, I'd say that if you are used to a single-barreled gun, auto or pump, an O/U will be easier to use well. If not, either double barrel should work well.
I was raised with single-barrel guns (single shots) and, while I have and use several SxSs, I shoot my O/Us noticeably better.
