Having worked myself as a constituent contact in a Congressional office, I can tell you from experience that you are exactly right. Someone who's willing to hand-write a letter and pay for a stamp is seen as a more motivated constituent -- more likely to care enough about the issue to have it decide how they'll vote in November, in other words -- than someone who makes a call; and the caller is more motivated than someone who clicks "send" on an email; and the emailer is more motivated than someone who signs a Web petition.

For a long time, in fact, this mindset meant that a really effective way to "hack" the Congressional contact system was to send your message in a telegram -- because if you were willing to pay to have a messenger run your message across town and deliver it by hand, you were assumed to be very motivated indeed!

(I doubt that's true anymore, of course; not least because I'm not sure you can send telegrams anymore. But now I'm wondering if you could get the same effect these days by sending your message via FedEx's most expensive service...)