Part of the conclusion just doesn't make sense even though i agree the Evo is so much better.
As a potential upgrade, the Z400s is a fairly poor prospect, largely because of its price. In terms of value, Samsung's 850 EVO just wrecks every other company's attempt to compete. With the 128GB Z400s at $45 and the 120GB 850 EVO going for $68, we have to recommend Samsung's offering.
Unless my maths are way off, which it very well could be, the EVO is 62% more/GB. For a gaming disk, the Z400 is just as good as the EVO, and i suspect for a thin ultrabook with a "U" cpu, the cpu will be the bottleneck long before an SSD would be a showstopper.