Well I'm gonna have to disagree with you chaps, as we made the opposite decision when buying ours. Depends if you need 7 seats or not, and the type of driving you do. We bought a Scenic, for just that kind of money, after trying several of both.
Reasons were:
Scenic slightly smaller so easier for the missus to manouvre.
Zafira felt more bus-like to drive - steering wheel angle is flatter and further away from driver's seat.
Scenic had 5 proper seat belts (earlier Zafiras only have a lap belt in the middle of back seat).
Zafira got 3* NCAP, Scenic got 4*
INteriors of Zafiras are a bit like all Vauxhall's - dark & dingey.
Seats are better for long trips in the Scenic than the Zafira, and Zafira rear seats are horrid to sit in as they are so thin (need to be to allow them to fold into the floor). If/when one of us has to sit in the back with the kids, the seats are as comfy as the front.
We got a 1.6 16V Privelege +, which has Alloys, ACC, 4 x leccy windows, twin leccy roofs, 6xCD, half-leather seats (easier to clean when #1 daughter hurled all over the place a few weeks ago!) etc.
Another good point for my missus is that the 1.6 Scenic is relatively low geared (4000rpm in top = 82mph) so it feels nippier to her - she never revs beyond about 4000-4500, even if overtaking

and from my point of view she's less likely to drive along at 50mph in 3rd gear like she does in the 9-5

- also means it copes with hills on the m-way better than if over-geared, and fuel economy still OK. We got 38 on mixed roads coming back from the Lakes, including some pretty steep climbs.
But - if you need he load-lugging ability or the extra seats then it's probably the Zafira you're after.