The choice between Snapcaster and Mission Briefing is highly dependant on your build and what role you want/expect it to perform. I do run two Gifts Ungiven/ Thirst for knowledge in my deck since I'm heavily relying on the Salvagers/Pyrite Spellbomb combo, so, Mission Briefing might give me more mileage than Snapcaster. Although, Snapcaster doesn't get hit by Flusterstorm, so, there is that.
My only gripe is the double blue for Mission Briefing.
This card is definitely not easy to evaluate without further testing.