Switching to Asirra or a better captcha?

We've been getting spam account creation and some accounts that appear to
be bots<http://docs.webplatform.org/w/index.php?title=Special:AbuseLog&wpSearchFilter=9>
.<http://docs.webplatform.org/w/index.php?title=Special:AbuseLog&wpSearchFilter=9>
I've
noticed that we have been using
this<http://www.mediawiki.org/wiki/Extension:ConfirmEdit#MathCaptcha>for
registering a new account. I was hoping maybe we could switch
to this <http://www.mediawiki.org/wiki/Extension:ConfirmEdit#Asirra>instead.

It's a little ugly and you might have to beautify it to make the it match
the site well but it works a lot better for preventing bot accounts. Bots
have a hard time identifying cats from dogs. They get past that math
captcha pretty easily though. Does anyone have any better captcha ideas?

I'm curious if anyone knows something that would work better or if you guys
thinking switching it is necessary.