In the year 2009 snail mail is grossly inadequate.
1. Does it prove anything? No. Sign something with a private key, that is proof.
2. Is it better for record keeping? No. Fires, floods, and rot are all enemies. Storing it digital allows one to replicate a lifetime of mail in 10 seconds.
3. Aesthetic? Only if you have a strong packrat or nesting instinct.
4. Cheaper? No. Not by a fucking longshot, not even close.
Take a one dollar bill out of your wallet, burn it, and send an email.