If you are letting prudence reign and you are dressing for a swim, or a rescue, bring the dry suit. The Salmon is a cold, relatively short and steep drainage with very little flat water to warm up the water temps. Not sure one can make a generalized statement that CA rivers are warmer than CO rivers, since snow melt is snow melt and the melting point out here is the same as it is in CO. Better to have it and not need it, than need it and not have it! As stated above most things are doable with either piece of gear and air temps look to be friendly so I guess you should just decide what you are dressing for. I will be in a dry suit this week on the Salmon. Bummer you can't bring both...
Great time to be on the Salmon for sure! If you are interested in any of the side creeks (which are plentiful and bitchin) then definitely bring the dry suit. As I can assure you that the water temps on the creeks will rival the coldest CO creeks. Have fun! Special place indeed.