I can't say specifically for the Frog since I got the 8K version & the alum plate it has is not leaf spring. Although overall I'd say the full alum leaf spring plate will for sure give you a more consistent & softer feel over the whole board, also more consistent sound. The FR4 half plate will give a less consistent feel & sound between the alphas & everything else just by it's nature, also I suspect it would have a stiffer feel overall since it does not have the leaf spring cuts. IME half plate is best used with a boards that have stiffer mounts, as at least the alphas will feel a little softer. The trade off there is inconsistent feel & sound between the alphas & everything else IME.
Edit: I should mention we're not talking huge inconsistencies when using half plates, the alphas will feel a bit softer & sound a bit deeper than the switches that are in the plate part is all. Just was pointing out that by it's nature a half plate will introduce inconsistencies between the alpha plateless section & the plated sections.