If you need lots of power to run your membership site, you might want to check out MemberSpeed. It is used to run several high volume sites and offers a lot of features.
There are plugins that allow you to build any membership site you can imagine.
Finely detailed control of artcles, member levels, products and more are provided.
You can also include a full shopping cart system on your MemberSpeed powered site.
All this power and flexability comes at a cost. MemberSpeed is not cheap, but they now offer monthly plans as well as complete purchase options.