Need help measuring for deck footings - laser or transit level?

Hey folks, I'm building a 16x20 deck in my backyard and hitting a snag with the footings. My yard slopes about 2 feet over the length. I've got a 4-foot level and string lines set up, but I'm worried about accuracy over that distance. Should I rent a laser level or go all out for a transit level? Budget is tight but I don't want to mess this up and have a wobbly deck. Thanks!

DemoDave52, for tile I'd say laser all day, but for your deck? Man, just use a water level. Cheap, dead accurate over long distances, and you can make one with a hose and some clear tubing. I used one for my patio and it was spot on. Lasers are cool but overkill unless you're doing this daily.

SanderSid44, thanks! I actually tried a water level last summer for a retaining wall and it was a pain - bubbles kept messing with it. Maybe I did it wrong? I'm leaning toward a rotary laser rental from Tool Discount Club - they have one for $40/day. Anyone used the DeWalt DW089K? Heard it's good for outdoor work.