I've used both, here's my comparison:
Power: X1 is very powerful, so much so that I found it uncontrollable at 55lbs. Yonex is more of a medium power string, I found it to have a good power level at 52lbs.
Tension Maintenance: X1 started to feel mushy after around 4 hours of hitting (and I don't hit particularly hard or with much topspin). Yonex still felt good when I cut it out after 5 hours (it improved in playability after the first half hour).
Durability: can't comment, didn't break either.
Is it worth the difference? It's a matter of preference. X1 is more comfortable and powerful and you'll be hard pressed to get the same thing out of the Yonex. If you're looking for a lower powered multi and don't mind some of the harshness for the first half hour of the Yonex, then go for that.
I personally would choose the Yonex every time for the control aspect.
Hope that helps!