Redirect - how expensive(time) is it?

redirect versus API interface

[mkstreet]

I'm working with two different sites.

I want my users to login, enter some information to the other site service, and then come back to me.

I have one option of redirecting to this other site and then they redirect back.

The other option is doing all the presentation myself and sending a SOAP message to the API of the other site.

Opinions I have heard say that the redirect will be "slower".

The question is: how much slower? how can I figure out how expensive(time) this will be so that I can compare these two approaches?

Any ideas would be great.

thanks in advance.