Twitter Follower vs Email Subscriber.. How do we understand both types of individual from a marketing angle?

Effectively the same amount of attention span in a subject line of an email to that of a tweet or DM on Twitter.

But how do we compare somebody who has opted in to receive emails from you or someone who has opted in by following you on Twitter?

Can you compare the marketing value, ie: 100 Twitter followers against 100 Email subscribers (assuming that none are spam/bots of course).

Im interested in seeing the responses.