I recall once talking to a store clerk about solid vs. stranded wire of the same gauge. He said stranded wire was better because electricity travels on the surface of the wire, and stranded wire has lots more surface area. That's total BS !!!
Actually it isn't total BS, it just only applies to very high frequency currents. The effect is meaningless to anything in model trains or general household use, but it does exist.
As far as layout wiring goes, use whichever you prefer. Solid is probably easier to get a clean strip and to attach to things, where as stranded is easier to fish through to the location.