eBook against Digital Services which one will get more sales when it comes to a niche like SEO?
For fields vast as SEO. You must know first what your audience want. You can't compare these things. Both can do well if they are targeted towards the right crowd.
It depend solely on the audience. Both can convert well if you handle them neatly.
I think its a ridiculous theory. Both can work great. You just have to concentrate on the item in which you want to get sales and make you more money.