Thematically speaking, Ambushes last shortly and a permanent version just doesn't make sense to me.
The first idea makes much more sense thematically, but creatures can be Unburrowed and have their attack doubled for free already. In a sense, it's more of a one turn mass Momentum card than an attack boosting card (unless, of course, you had envisioned the attack double after the initial attack double) and should cost less until more Burrowing creatures are introduced into the game.
That said, are the Unburrowed creatures Burrowed again after the turn? Such a hit-and-run tactic could merit such a price?