Maybe you'll find the awnser if you'll try both for a while. And if you don't, practicing multiple disciplines is a good thing imo!
And with the lower legs: make sure your stirrups are the right length, don't squeeze with your thighs and ride from your core :)
No stirrups will help keep your lower leg still!!
Any reason why you can't do both? Is your horse better at one than the other/does it excel in the showjumping and lack in the XC and dressage?
Or maybe you can do one now and switch over.
I'm always changing between dressage, jumping and Eventing to improve my skills, but my main goal is to be an eventer.
But I have asked myself the same question if I should just do straight jumping, but I love XC too much and really enjoy Dressage too.
I found riding with no stirrups helped with my lower leg because you have to hold yourself there with your lower leg and seat