Chris,

My apologies for the long response delay. I thought I was getting automatic notification of submissions and it turned out I wasn't! So I didn't see this question.

There is not really a simple answer to your question that applies to all applications for web conferencing. On a purely fiscal basis, GoToWebinar wins for letting you host as many sessions as you want, each one with up to 1000 people, for $99 a month. Hard to beat that. Of course you are running everything through screen sharing, and if you know you have audiences on slow connections, that is not an optimal approach. It also won't work when you want to start showing video.

Adobe Acrobat Connect Pro has the flexibility to show all the things you want to include, but it starts getting expensive above the 100 person per session level. They also charge extra for the registration and attendee management functionality you expect in a a large session product (and it's pretty basic).

ON24 has been handling very large events for a long time. They can show slides and speaker video as well as video clips. But they don't support animations in slides, in case that is important to you.

A newer company has caught my eye lately and I am running some additional tests. It is called omNovia, and I have had favorable first impressions. It is Flash-based (like Adobe) for fast universal access. It's worth taking a look at.

I can manage to find a weak point about any technology you look at... Nothing is perfect for all uses. So it's a matter of balancing out your priorities and budget and then grilling the vendor about the things that are most important to you.

By the way, iLinc can handle the audience sizes and content pieces you mentioned, but it converts slides to static images (like ON24) and has a download for participants when they first attend. Not as large as the other guys, but still perceptible.