They should both be checked. It may slow down your ability to send and receive e-mail, but it is a good idea. Here is why:
Some viruses have the ability to not only infect your computer, but also attach to your address book and send a copy of itself to everybody in your address book. So if you don't have those checked, you could become infected by a virus and it could still replicate and send itself out via your e-mail account. This will piss off a lot of people because they might think that you purposely sent it! So you need to scan incoming AND outgoing e-mail. Keep those boxes checked!