So I saw a mention from the DMZ about me and this, so I'll chip in.
I guess the answer partially depends on why StackExchange do sponsorships...
I'd guess that one reason is to help boost awareness of the SE network and specifically Sec.SE. In this case then I'd say that sponsoring OWASP events in general should be a good option.
OWASP events tend to be primarily attended by people on the technical side of application security (both security practitioners and developers). I'd say that the attendees are a good match for the kind of people we'd like to see contributing here, so raising awareness of Sec.SE with OWASP attendees would be good from that point of view.
There's also the aspect that @RoryAlsop raised, which is that OWASP are a non-profit security organisation who depend on sponsorships and memberships to raise funds for projects and conferences.
Quite a few of their projects are well referenced and used in the security community in general (the OWASP Top 10 in particular) and get quite a few references on Sec.SE (210 from a basic search), so hopefully helping to fund them through sponsorships helps to improve the overall state of the AppSec industry...
Is that helpful?