I've only done one race so far (40 minutes, beginner). My lungs felt like they were dying, but that started two minutes in, so the last ten minutes didn't make much of a difference. It flew by so fast. It seems like a 30-minute race wouldn't be enough time to get into it. I want to get my money's worth.
What irks me is having a "women's race" with levels 1-3 all together. That's a wide range of abilities, and strikes me as unfair to top-level female racers. It also shunts women off to one side, so there's a bunch of men's category races -- masters, 1s, 2s, beginners, Clydesdales... -- and then one other race as a catch-all for the women (beginners excepted in the situation you mention). I like that in Oregon A women and A men (Oregon stuck with the letter system this year) race in the same time slot.
It still irks me that my number reads "WO BEGINNER" whereas the men's just read "BEGINNER". Talk about otherizing.
Anyway, tangents aside, I vote for the longer race.
2011 Scott CX Team/Koobi Xenon
2005 Bike Friday NWT/Selle Royal Lookin
2001 Bianchi Giro/Terry Butterfly Ti
1970s Centurion Le Mans/Ancient waffle saddle