Guild Wars 2

I will get both of them, but which one should I get 1st ( more useful ) ?
Spirit Weapon Mastery ( -20% recharge ) or Improved Spirit Weapon Duration ( +50% ) ?

So with the first one, you will have your spirit weapon (just taking the bow, lasts for 20 seconds and has a 60 second recharge) 30/90 = 1/3rd of the time, with the other you will have 20/(20 + 48) ~ 0.294 of the time.

So by 6 1/100ths of a second, you should get the spirit weapons mastery trait first

On the other hand, you could look at the prerequisites. Inspired Virtue is more useful than Zealot’s Speed in most scenarios, so it might be worth going down the Virtues tree first.

The key here is deciding whether you want to summon it more, or want it to last longer.

SW Mastery (-20% CD) is better when you’re killing mobs fast and moving from group to group a lot.
Improved SW Duration (+50% duration) is better for long fights.

And as Urthona mentioned, the Inspired Virtues minor trait is significantly more useful.