E-Mail vs Snail Mail Submissions - Why Content Differences?

Most agents won't accept attachments.

I suppose they ask for more information via snail mail so that they don't have to snail mail back to ask for the chapters.

The agents I've checked out ask for the same information whichever way it's sent.

Yeah, I think Carmy's nailed it. With emailed submissions, an interested agent can request more and get it instantly. If the query was snail-mailed, it could take a week or more to be able to read further.

If given the option, I'd always email. Not only do you save some trees along the way, you're more likely to get a quick response. My one snail-mailed query took 13 weeks to generate a response. 3 out of 4 agents responded to my emails within 24 hours.

e and snail

When given the option, it's usually best to snail mail. According to the large majority of agents I know and have talked to, snail mail still makes a better impression, and still gets treated much more seriously. Which is one reason agents often ask for more with snail mail.

I have to disagree here (with out hopefully being disagreeable!)
I had much more positive and speedy response via email even though my agent stated she preferred snail mail.
It is not for every situation. I think many genre projects may be best sent via regular mail.
If you are querying a large agency like William Morris you get to the agent's assistant first using email rather than that big pile of slush in the mailroom.
I think email also works best with large agencies and high concept premises.
That being said, if you have a great query and terrific first pages you will get attention regardless whether you email or use snail mail.
JMHO

Biggest advantage of snail mail is you get to send some pages and, unless the query letter stinks, the pages will get looked at and this may get you over a less than perfect query letter.

Unlike Orion, I had a much better experience with snail mail. After hearing that a lot of agents prefer snail (even if they accept email), I decided to play it safe. Plus, with email, many agents won't respond if they're not interested--so you're left wondering if they received the query.