Ditto what OF says...two different guns here for two different purposes.
The main problem with the OM 357, in my opinion, is the barrel length. Basically, that gun is (or has the potential to be) a handy-sized packin' pistol but for that purpose, it really needs a 4.68" or at most 5.5" barrel. With the long tube, the "easy packin'" part of the equation is...lacking, if not entirely gone.
Now, if that OM has a flat-top frame, BUY IT. Period, end of discussion, if only for trade goods. "Flat-top" means the rear sight and topstrap are flat rather than the sight being a bit raised and two "wings" on either side of the rear sight (which on a flattop is still adjustable).
Another valuable difference on some older OMs is an "XR3" grip frame instead of "XR3-RED" which is what the 45 will have. The XR3 is almost identical to the Colt SAA grip and with fitting can take Colt grip panels (so long as they're two-piece versus one-piece). Being smaller, it is particularly well suited to "pinkie under" SA shooters (strong-hand pinkie slung under the grip). The transition from XR3 to XR3-RED happened after the flat-tops vanished but long before the New Model, so there's a small number of "transition guns" with the modern-type rear sight but XR3 grip frame. In this case, spotting which grip type is on the OM is easy: compare it to the NM. If they're the same, then you have a late OM on your hands of no particular collector value...you could chop the barrel even with the ejector housing (4.68") down the road, in which case that gun would really make sense.
So flattops are VERY valuable, guns with an XR3 grip but otherwise post-flattop are less so but uncommon enough that it's probably still good trade stuff and you might like the smaller grip for yourself.
The NM in 45 with that long barrel will get up a MASSIVE head of steam
. That'll make a superb hunting gun and with basic mods (Belt Mountain base pin, cylinder throat job and maybe springs, well under $100 for the lot) you'll get damned fine accuracy.
