What I've had more success with are smaller "teaser" emails leading to a long copy salesletter or web page. Why? Three reasons:

1) Less content in the email is easier/faster to download and reduces risks of triggering spam filters. More content means more words that can trigger the filters. (Larger emails have a much smaller "open rate" than smaller ones. For example, I really love Marcia Yudkin's Marketing Minute or Markus Allen's $10,000 Tip of The Day. Their emails are 2-3 paragraphs, often with a URL leading to a web page.)

2) Causes curiosity and also adds a sense of value -- why? because if they have to go online to a "secret" or "private" page (so to speak) to retrieve the content, it adds a sense that they are privileged in getting such content. It's not easily accessible and projects an aura of scarcity. Whereas straight, full content emails are "free" and thus lessens the perceived value somewhat.

3) Small emails also tend to "pre-qualify" reader before hitting the promo. If they read the email and then visit the web page with more content, they can be put into a favorable frame of mind and pushes only those people who are truly interested in what the content (or topic) may offer. We tend to track not just open rates (achieved only with HTML emails) and clickthroughs, but also sales per clickthrough (rather than sales per campaign). Sales per CT's are higher with smaller emails.

Plain "html" emails work best over plain text and html-rich. Meaning, I use html for formatting, colors and adding strong borders (which increase readership), but no graphics, scripts or forms.