Subscriptions are the future and here to stay (Think of Spotify, Netflix, etc.) Consumers are going to be more in the demand of lower monthly priced subscription with the promise of getting continued service and fresh content.
Subscriptions also mean more money for you. For example, you sell a $25 ebook to 100 people your total earnings are $2,500. Sell a subscription to 100 people at $10 month, that's $1,000 per month for the length you can keep them paying. 12 months is $12,000.
Should I sell both?
You can sell both, but the harder it is for your customer to determine the difference between the products the less successful you will be. You will want to make sure the offerings and content inside a differentiated so your customers know what to buy.
We'd recommend getting your audience on a subscription than a one-off purchase.