Ask HN: Hosting an event on Heroku, App Harbor, or EC2

At work, we're considering hosting for a one day event that may receive considerable traffic over four hours (we hope for 100,000 visitors). The site will most likely be .NET due to a tight deadline and us knowing .NET better. Here's a quick rundown of the simple things the site needs: one form that will be used frequently, all text fields; one comment form used less frequently, also text; image upload used infrequently; image and comment feed for moderation; another feed of approved comments and images; probably to send about 30,000 emails. So it's simple enough, and if we host assets on S3, we should offload a lot of the traffic to S3.

Our likely candidates are Amazon EC2, App Harbor, or some other dedicated server. I'm not sure how good App Harbor is. I hear great things about Heroku but have no idea what traffic it could support. If there is a strong argument for Heroku over everything else, I'd be interested to hear it. Our imperative is to keep the site up and running well during the whole event.

Any tips? This is my first introduction to such a large volume of traffic. I'd greatly appreciate any advice.