Ha ha, we were always complaining about the bouncing around on short waves on the North Sea.... later found out that the waves in the Caribbean
are just as short and bouncy (not at all like the postcards!) and now you tell me that it ain't no different in the Med ;-)
I think the two most important factors are the length of the boat and the weight of the boat. The longer and heavier, the less bouncing. As 50' is a pretty good size already, you should look at the weight first, plus hull
Weight: the Jeanneau
51 was a charter
boat? If so, you don't have to worry much because once you load everything aboard for cruising on your boat, your weight will be way up!
shape: the bow can slice through the waves or "slam" into them. Our bow is without overhang (maximum waterline) and with a very fine entry angle, so it tends to slice through the waves pretty good. However, over a certain wave height, this effect is becoming less and less. Also, this means that we are very shallow draft
at the bow (and less buoyancy), often coming out of the water
and slamming back in. So, it's not all paradise either. We need speed for lift
at the bow, unlike boats with overhang and a bigger entry angle which get much more buoyancy as the bow immerses more. The shape of our hull would not be so comfortable at 50' length I think; The Sundeer 56's are also much slower, more difference than just the length. Probably, some hull shapes work best within a range of length of the boat while others are more relaxed.
We sailed Jeaneau's too, but not the 51. I tend to feel that they are more bouncy than Beneteau's. But I wouldn't know about the difference between the versions of the Beneteau