Ask HN: What license for our ad server project?

I've been working on an ad server (that collects and randomizes ads from different sources, displays them on your site, and collects statistics). We can't decide what to license it under. GPLv2? GPLv3? Affero GPL? MIT??

My initial reaction was to license under 'GPLv2 and up'... I have some qualms with v3 but I don't want to limit anybody's use based on that. But it would suck if someone took it, improved on it, and made a lot of money off of it (say by running an ad hosting service like OpenX), and never contributed their changes back to the community.

But I'm not sure I'd use a project under Affero (if it weren't mine). Argh what to do?