If you're set on a Greys GR50 get the 5/6. I think 300 grains might be a bit light for that rod, but since there's no grain window listed for that rod it's hard to tell. You might want to consider contacting Greys with your question.
I have a couple 5/6 switches from other manufacturers, and they like a scandi of 325 to 350 grains and a tad heavier for a skagit. Of course every rod is different and line ratings for switch rods are all over the map, some are rated as single handers while others get two handed line ratings. It's hard to know unless you get a good grain window for a rod.