Tell you what, in the last six months in particular, prior to all the panic buying, I've turned two S&W Shields and two Ruger LC9s' into HK's and Sigs with very little $ put into the deals. Right place at the right time.
As far as a 9mm cylinder, I've never priced them but if you can get one for the revolver you decide on I see it as a definite plus. First, a +P 9mm in a revolver is going to be more potent than using .38 Special. Not as potent as .357 magnum but a good +P 9mm is going to be a lot closer to 357 than it is to .38. It will give you options and as I always say, options are a very good thing. And it would be cheaper to practice with using 9mm than either .38 or .357. Overall I don't see a downside.