While both are solid, the x60 line is better under transient loads. The one issue with the x50 line is that it doesn't do so well in that department. It's not huge in most cases, but it is the one weakness in an otherwise perfect power supply.
The x60 series addresses that issue. If you can get an x50 series on a great deal, it's still worth doing (I did it). However, if the X50 and x60 are similarly priced, get the x60.
i7-4790K, GIGABYTE Z97X-UD5H-BK, 16GB GSkill PC1866
Corsair 650D, Seasonic 1Kw Platinum
WD `Raptor 600GB, WD Black 1TB, NEC 7200 DVDRW
STRIX GTX 980, X-Fi Ti, Dell 2408WFP