You'll probably get more email subscribers than RSS subscribers. Everyone & their brother has email, doubtful everyone cares about RSS.

IMO, use the same content, just a different deliver (email vs RSS).

Even Feedburner (Free) converts a Feed to an email, each time you post a new blog post Feedburener will automatically send out a new email to your email subscribers.

Feed Validation AND Make sure that the feed is properly formed. Some RSS readers and aggregators will be unable to compensate or read an RSS feed that is not correctly formed. There are both web-based and desktop feed validation tools available that can help determine whether an RSS feed is properly formatted or not. After you edit the feed, be sure to spend a few minutes validating the feed.

Properly Promote Your Feed In order to increase the number of individuals subscribing to your RSS feed, take the time to properly promote the RSS feed. Just as a website requires promotion and marketing, so does an RSS feed. Here are a few tips for promoting your feed