Have you considered,,,
Have you considered a
S&W Model 43 C?
Since it is a J-frame S&W like your wife's 642s,,,
It would be a perfect rimfire understudy.
Nothing wrong with the Ruger,,,
I've fired one several times and it's nice,,,
But I have also fired a 42 C and really liked it a lot better.
Just tossing that out there.
Aarond
P.S. I agree with your thinking of having a rimfire understudy to a centerfire handgun,,,
I'm a pretty decent shot with both my snubbie and 3" model 36 revolvers,,,
I attribute that to a lot of cheap practice with the Model 34 rimfire.
I believe one must shoot both guns though,,,
Typically I will shoot 50 rimfires,,,
Then 5 or 10 centerfires,,,
Repeat many times.
.