Ok, let's assume that, shall we?
Dave played first, meaning he started with 7 cards in his hand. On the first turn, he dropped to 5. On the second, he went to 6, then to 4. On the fourth turn (the turn he draws and lays a Mountain), he remains at 4. Scrolling with 6 cards is hardly a guaranteed situation, I would find it more likely that he would instead _burn_ the Spike, via Incinerate or some-such, neh?
Assuming he does so, the life totals go to 18/20 (Brian/Dave, for those who forgot). Current board is 3 Mountains, one Jackal Pup, one Mogg Fanatic, and one Cursed Scroll (active) against 2 Forests, 1 Swamp. Yes, I would say Dave now has an advantage. But it should be noted that at this point in the game, Brian has quite a bit of manuvering room, at 18 life on turn 4 against Deadguy Red. So, again, this game is in the air. Brian still has 6 cards left in his hand (started with 8, to 7 from land, draws to 8, land and Wall, going to 7, draws, land and Spike, going to 6).
Basically, I'm just saying that it's unfair to say card advantage is everything.