Also, the LCR is 2oz. Lighter.
To add on to this, that is only true with the .38 model (and the .22). The .357, .327, and 9mm models use a stainless steel frame. While it is more obvious to go with a .38 to .38 comparison, some people forget that note and end up with a heavier gun... since you can shoot both rounds out of a .357.
I’ve been shooting a 642-1 since 2009. Few years back, I got tired of the .38... and wanted to move to the 9mm. Came down to two options; buy a 9mm LCR or convert my 642-1 to 9mm. My father had a .357 LCR, so brought it to the range to try out...
Wasn’t a little heavier, but A LOT heavier. Trigger, which to be honest, is identical to the 642 in design (DAO). Very different in function. But if you are used to the J-frame DAO, you aren’t going to like the LCR trigger. Going to have to really put a lot of training on it, which is a requirement for small revolvers like these. I opted to convert my 642-1, being I’d lose a lot of gear (holsters, etc), money (gun was refinished in NP3... would never make that back), and most importantly, years behind the gun. I like the lighter 642 over a 9mm LCR (pretty much the same weight as the revolver I was comparing), but not transitioning to a much different platform was my main reason.
Now, if the 9mm LCRx was out when I was making the decision... I’d probably have gone with that. Getting SA, with an exposed hammer, would have been a nice option. And would have been one that I would have justified the change.
While looking at technical specs is important, you really need to get the two guns in hand and try them each out. At least dry fire the two (models you want) to get an idea on the triggers, and consider you are going to be putting some rounds through either to train. Try not to pick one based on initial impressions, but where you think you’d be after getting used to it. The LCR trigger... either you are going to like it or hate it. Is it lighter? Yes, but with a lot of distraction through the pull. I personally prefer S&W triggers over it. If you find a range that rents, you’ll likely find a .357 version of the LCR... and probably an all steel J-frame (640, probably in .357). I’d try to see if anyone local has a .38 LCR. Any traditional 642 would give you an idea on performance.