Verethragna is Offensive and Spharai is Defensive/Tank. They are close in my eyes in terms of performance and importance but Vere pulls ahead because of aftermath effecting each melee hit plus counters. It's easy to keep aftermath up as Victory Smite will arguably be your strongest and best WS option.
Final Heaven is also stupid and will never be used, You would use Shinji Spiral 5/5 instead if you went the relic route.
Vere only effects the 2nd hit of every atk and does not proc on dbl/triple/quad/counter.
If i were you i would make a 90 vere and a 95 spharai. Once you have 90 vere and 95 spharai.
I've had 99 Spharai and 99 Vere. The only time 99 vere will be better is when you are in voidwatch or zerging a monster that you can start the fight off with 300 tp.
And abysea.
Spharai Pro's would be Dynamis / Nyzul isle / old content such as sky / salvage / eijinhier (butched that)
If you basically zerg / vw / abysea then dont even bother making the spharai, If you plan to do a lot of dynamis / nyzul / salvage / einjin~~ then make the 95 Spharai before making your 99 vere.
Edit: If you play mnk over the top, like i did. I would advise you to have a Final heaven macro to make DBL light.
If you are unfamiliar with the term "dbl light" all it means is this:
X weapon skill + X weaponskill = Light + Final heaven = light again.