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.
I played 1 mission briefing and 1 snap caster in my build yesterday in a 24 man tournament. Entered top 8 as the #1 seed but lost to Dan Miller in round 1 of top 8 after taking him out in the swiss rounds.
Through all my games I drew mission briefing twice. Once it got pitched to FoW, though if I was able to untap without needing to pitch it, I had planned to use it. The other time it got put back on top with brainstorm and shuffled into the deck because I just didn't need it. 2 of my swiss rounds were against shops and I sided it out in both of those games 2 and 3. So all in all, I didn't really get much experience with it.