Going with Zabuza on both occasions mostly because of the location. More mist and Zabuza can trap Guts in the Water Prison.
@MrASSH0LE: Let me be a little more clear then. Is the armor enchanted under Schierke's magic or not? The former allows Guts to retain his sanity while retaining the powers granted to him via the Berserker Armor so long as he keeps his feelings of hatred in check. The latter has him going berserker.
If the former, then having Guts being bloodlusted is kind of redundant since he's always willing to kill those who threaten his life.
@MrASSH0LE: Then I don't see what having knowledge of Zabuza's abilities would matter to Guts here as being berserker under the armor's power comes at the cost of Guts being capable of devising tactics and strategy on the fly in battle. The problems for Guts in this match is that his arsenal of weapons and abilities are limited to close and mid-range combat, while Zabuza's capable of engaging his foe at all ranges. In addition, Guts' attacks lack the wide spread amount of damage and effect they would have on the battlefield that Zabuza's have potential to cause via his water jutsu.
Granted, direct close combat against Guts would be fatal for Zabuza. But the Missing-nin's also a competent tactician capable of devising means of counteracting his opponent's abilities once he gets a feel for their abilities in battle, giving a jounin-level ninja in the form of Kakashi difficulty during their fights in the Land of Waves arc. Via his water jutsu, Zabuza could fight from a distance via use of water clones or milk ranged attacks via Water Dragon Bullet and Great Waterfall. The island terrain is more a boon for Zabuza here as he can use the water in the area to help further strengthen his jutsu.
Overall, I'd rate this match 70/30 in Zabuza's favor. If the Missing-nin gets careless and tries resorting to close combat, Guts would easily slaughter him. But if he plans carefully and engages Guts through deception and ranged combat, then it greatly increases his chances of victory here.