redirecting traffic to the new site - best practices

We have a web site up and are moving to a new site, with a more modern design and some jQuery/ajax features for better usability. The old site is on IIS6 and the new one is on IIS7.5. The old site is on domai开发者_如何学Gon.com and the new site is on beta.domain.com. I want to test out how the load will affect the new server, so I want to begin sending users to the new site slowly. I want to start sending every 20th user to the new site and save a cookie so that a user that was directed to the new site will always be directed to the new site and a user which was directed to the old site will always be directed to the old site, as long as we don't change a key and "reset" the system. I was wondering where would be the best place to implement this behavior / logic?

How about in global Application_BeginRequest.. as for the best way to do it, by far the easiest way would be a simple Response.Redirect and I can't think of any real downsides. No reason not to just keep it simple.

Application_BeginRequest seems to be the best place to put such logic for me. However I'd consider some issues related to SEO, I don't know whether if matters to you. You may decide to send 301 Moved permanently or 303 Moved temporarily HTTP response codes. Also I think it would be a good idea to detect a search engine crawlers and always direct it to the same version, no matter if old or new, as it might be a little confusing to them when they get different content form the same address depending on crawling server.