Red dot mounting question

Hi all.  I have a RO, and I am getting a Nelson .22 conversion for it.  If/when I go Red dot, would it be better to have a sight for each slide, or use a frame mounted site using just the one sight for the 2 calipers?  Pros,cons?
Thanks, Kurt
Ps please recommend a sight too.

Sight for each slide, or do the dot for the Nelson and Irons for the .45.

Grip mounts don't really solve the problem for using it for both a .22 conversion and the .45 as the zero is not the same.

Thanks Rob, that makes sense.  I wasn't thinking grip mount, I've seen a frame mount-frame has to be drilled and tapped for it.  Kinda thinking RD for the Nelson.

frame mount is the same problem.  The 2 uppers just don't have the same zero.

There is one frame mount that would solve the problem it's by David Sam's but to install thats like 500 bucks 

I would just use ribs on both and use a two peice guide rod to make swapping uppers easier.

Same problem with Sam's mount.  You would have to re-zero when switching top ends. Slide mount is best way to go.
Jon

If you wish to mount a red dot (Such as Burris FF III) on the slide of the RO to replace the iron sight, note that Springfield uses an unusual cut.  Evolution Gun Works makes a mount that fits as part 49413.     Mac