I'd have to say neither. :P
might have scans from one of the character books; she's still working on the website, though, so it might not be up yet. If you get this page, look at his sandals. They're about an inch high (at least), and have a small rounded section cutout on the instep.
Zori are solid all the way across the bottom and geta have a separate heel. They're still connected, though, so I'd lean toward zori. His clothes look similar to traditional Japanese clothing (as does a lot from that series), but none of it is exactly right - he's taken liberties with the designs.
However, if you were to puchase a pair, I'd probably say go for the geta like the 3rd pair on that website. They're more of a women's style, actually, but that's ok - Haku's pretty girly. I suggest this pair b/c I think they should be wood, and finding zori made from wood would be rather difficult. They're usually leather or vinyl.
I have the same book, so I'll scan the pic when I get home from work tonight, which won't be for about another 10 hours (haven't left, but don't have time right now to set up the scanner).