I refuse to buy a revolver with plastic sights.
Then think of them as throwaway like Savage synthetic stocks. They are fine for people who don't care and Ruger didn't want to waste money when the people who are particular were going to want a specific sight anyways.
I think the "barrels are different sizes" bit has been looked into several times and ALL the companies who are making both/convertibles are using the same barrel blanks for both. For many rifles also. Maybe a company like Freedom Arms isn't. I called around a bit when I was buuying mine and that was pretty much the story I got everywhere.