I wouldn't suggest base64 encoding for messages that are mostly text (plaintext, html messages), but it's just dandy for attachments.
Debugging something in an email message encoding in quoted-printable simply by viewing its source is doable sometimes. If it's in base64, not so much. I believe the size of your message would also be less using quoted-printable, rather than base64
also first time i was sending sms messages and not using ascii trying to determinate the message length was fun. and by fun i mean gsm 7bit annoying.