Putting together the two, I am only seeing about a £135 difference, based on the two builds below... that seems pretty good, so why WOULDN'T you go for X99 in this case to give you that bit more future proofing? I know you can spend an absolute fortune on X99, but do you need to? And looking at the below, would Z97 still be preferable?
Option #1: X99 - total cost = £632.93
MSI X99S SLI Plus
Intel 5820K 3.30GHz
TeamGroup Elite 16GB (4x4GB) DDR4
Option #2: Z97 - total cost = £497.93
Gigabyte Z97X-UD3H-BK - i7 4790K CPU & Motherboard Bundle
Kingston HyperX Beast 16GB
Option #1: X99 - total cost = £632.93
MSI X99S SLI Plus
Intel 5820K 3.30GHz
TeamGroup Elite 16GB (4x4GB) DDR4
Option #2: Z97 - total cost = £497.93
Gigabyte Z97X-UD3H-BK - i7 4790K CPU & Motherboard Bundle
Kingston HyperX Beast 16GB