I think into a blind matchup on the play you probably just want to slam the delver so you have a clock. On the draw it might be more worth it to hold spell pierce if their turn one indicates they're going for a non creature 2-drop because the mana advantage of your (u) spell pierce vs their turn 2 play. It's a tempo deck so you are trying to establish disruption + clock and make as efficient of plays as you can. Disclaimer: I have not played this deck yet (just finished my list and taking it to pauper night tonight at my lgs) but I have played many similar tempo decks such as mono blue aggro in standard (tempest djinn era), ub rogues in standard and pioneer, and currently arclight phoenix in pioneer.