Type: All-foam Firmness options: Firm, Extra Firm Height: 10" Sizes: 14 (twin-Cal king and RV sizes) Sleep trial: 120 nights Warranty: Limited Lifetime Price (on sale): from $599.25 at Plank Type: Hybrid Firmness options: Firm, Extra Firm Height: 13" Sizes: 12 (twin-Cal king and RV sizes) Sleep trial: 120 nights Warranty: Limited Lifetime Price (on sale): from $824.25 at Plank It’s true that the best firm mattresse s aren’t for everyone but if you’re the kind of person who eschews cushioning for unrelenting support, then you’ll appreciate Plank mattresses. These double-sided designs are the firmest mattresses we’ve tested, but is the Plank Firm or the Plank Firm Luxe right for you? One’s a hybrid and one’s all-foam but these mattresses have plenty in common, including that flippable firm feel. But there’s more than a few differences to consider. And as it’s more than firmness that earns a bed a place in our guide to the best mattresses , we’ll be conparing price, performance and design.…