I go for a big engine here - Lookout to trash down, Armory gains things, Throne Room for Shenanigans. It's a strong deck.
My opponent goes for Scavenger+HP.
On his 9th turn (he's first player here), he gets the final Hunting Party, his 6th. At this point, our decks look like this:
Let's break this down a little more. On that turn 9, I have a little more power than him, I can gain a bit more stuff, thanks to Throne Room and Armory. Island can mitigate the deleterious deck-bloat aspect I run into to some extent. In exchange for this, my opponent's deck is nigh-unbreakable. The key thing to note is, as long as he can get to $11 without drawing ALL of his Hunting Parties, he can Scavenger right then to seed his next hand with a Hunting Party. This gives him tremendous reliability. His deck will not break - if mine does, I'm dead. And though I am a pretty good favorite to not dud out on any given turn, I'm not terribly likely to make it through ALL of them, especially given that I need to make some magic happen with my Islands, and I'm somewhat limited on actions.
Now, part of me wants to cry "first player advantage", and indeed, though I lost by 20, I did have to turn for Province a bit early, and more importantly, that 1st-turn lets him get the 6-4 HP split here, which is pretty big. The problem with taking this mindset, though, is that even though I may be favored were I first player here, I would always be in a *better* position if I just play the simpler strategy. It doesn't blow my bigger engine entirely out of the water, and first-player might be a bigger deal, but it does have some advantage everywhere.