If you are talking about the boom, I've had both the X3 & X6. The only differences are the colour and the carbon back end in the X6. So the X6 is stiffer. Pop into the shop and give them the flex test to see if the X6 is worth paying the extra. Put the boom on the floor hold down one end with your foot and give the other end a jank with your hand(s). this should give you a rough idea of the fles difference and the extra responsiveness you'll get from your sail.
You might find that the X6s use a slightly higher grade of aluminium. The components are the same. The carbon nack end really does add to the stiffness
Oveall I thought the X3 was pretty good, both X3 & X6 have a continuous front and back end, so no joins or screws to corrode. The clamp head is now longer to spread the load on the mast. If you go for the X3 just make sure the extension pins lock in easily / don't foul and the back end slides in/out smoothly. I've come across a couple where the Aluminium outer tube was a little burred at the end and restricted the back end piece.