Membership & PayPal

Starting to set up Membership and lots of choices to make which I do not see in the documentation, unless I am not looking at the right documentation.

1. What's the difference between PayPal Express and PayPal Single payments. I have a paypal account, but I haven't a clue as to which to pick.

2. I have Network/MU installed on I see that Membership is listed both in the Network Admin and in the Site Admin. We are not using it (at least not now) as a MU site, just have it installed so we can use the nifty plugins at WPMUDEV. So should I be config in Network or in Site level? What's the difference, since most plugins seem to have different config options at the Network level than they do at the site level.