First, it looks better. For my taste Edocman css looks better.
Second, it is much faster. For example, bulk import 100 documents into both would result 5 seconds for Edocman, 20-30 seconds for DocMan. Both does the job without any problems, both stable but the joomdonation extension is faster.
Third, it offers more options, Edocman is more refined, let's say more accomplished and offers better modules. At the moment DocMan doesn't have a module for the "Most popular document" or "Most downloaded document" but Edocman does. I don't really understand why these modules were not available for Docman, but they informed me that these are on the way. Pity, they aren't available now.
All in all, both extensions are good very good, most importantly they are all rock-solid, runs without problems but for me Edocman suits better.
And one more thing: Tuan is really helpful, gave me a hand when I needed support and fixed the issue which was not an issue but I didn't know how to setup the component well. Thank you Tuan.
What is really important for me is a high-quality and fast support. They have it. Always quick and very helpful, so I can't say anything else but the support is first-class! If you, guys, find it important, than the RS Joomla team is meant for you.
Secondly, the component is also great, for me it does 3 things under one roof:
I can sell memberships with a lot of options. Great!
I can sell files. It serves as a document seller.
I can sell online tests by restricting the particular url so that noone but the subscribers can have access to it / them.
The payment by PayPal is quick and stable so it fulfills all my requirements. It is also true that I have not tested or tried other 3rd party membership components out there, but that one was the first I used and I think I stick to it because it has everything I want.