If we're talking on a hidden tang, you don't need either if it's fitted correctly. The only thing solder was ever SUPPOSED to be concerning a guards on knives..... was/is as a moisture seal, so that moisture doesn't intrude under the handle material and rot it from the inside out.
IF you are using solder, JB Weld, epoxy, or anything else to hold a guard in place, then your fit is too sloppy. The fit of the blade/guard junction should be so, that it looks like that blade it "growing" out of the guard. Again, if you have to use anything to fill space/gaps between a blade and guard, then the fit needs to the better.
Providing the guard fit is correct, and assuming a hidden tang, the adhesive that you use in the handle will fill and seal the guard/blade joint via hydraulic action, and once cured, will prevent moisture infiltration for the life of the adhesive.