If my priority was to be as strong as possible on Sunday, I wouldn't ride particularly hard on Saturday. There's a lot to be said for fresh legs. If I were really fit (which for me means averaging 200+ miles/week over a period of time) then I'd be OK with riding hard Saturday and expecting my best performance on Sunday too, but without those kinds of weekly miles, a hard ride on Saturday would diminish my performance on Sunday.

Your approach may depend on what else you can do the rest of the week, b/c if you want to ride a hard 50 mile club ride, you need to be putting in the miles at some other time in the week, and this may not be so easy this time of year.