I didn't consider a change in tire based on the OP's question-- I viewed it as "I have <tire model X> in a 700x38, would there be any advantage/disadvantage in going to <tire model X> in 700x42?"

There is certainly a tipping point where the tire gets too wide and too heavy, but I don't think that point is nestled between a 38 and a 42. I think the advantages of the 42 will outweigh any negatives in terms of weight or rolling resistance (which would well be lower.) Weight is going to be the only real drawback, but in touring terms, I think that's negated.

I guess at the end of the day, "Biggest tire that will fit" is my go-to-- but I find I tire I like first. [MENTION=199530]djb[/MENTION] is right on, if the tire is a clunker, a wider one isn't going to make it less of a clunker.