you may already know X18 has less offset than Big Bertha which means require a more 'proper' timing at impact. So i think it all comes down to your swing, if your swing is pretty reliable and can hit the ball straight or even draw a bit, then X18 is better for you. But if you are slicing most of the time, probrably better stick with the BB.

if you are thinking to use the irons to train yourself to get better swing, that to me may not be the best idea.. lessons from a golf pro should be the best way to do that.