Well, every time I see one of these posts that asks about trading one very hard to find S&W for another I have to oppose the idea....
In this particular case the OP has two Mod 27 3 1/2" guns and that justifies the trade concept a bit more.
How many times do we read someone's agonizing post that they sold or traded a very difficult to find gun as opposed to a statement like, "Boy... I'm sure glad I sold that rare S&W and I'm glad I don't have it back." How about...
NEVER!!
I can understand the desire to trade as money is always in short supply for many of us gun nuts. But I would try every other approach to try to buy the Mod 18 outright and avoid the trade. Both guns are substantially better than "money in the bank", and in our current economy that is a major consideration....
JMHO