Risk: What's the difference between Dialog A and S?

Hello together

I am aware of the basic difference of Dialog A and Service users (http://help.sap.com/saphelp_nw04/helpdata/DE/3d/3272396ace5534e10000000a11405a/content.htm). However what is the difference from a risk perspective?

I have a few questions:

1. Let's say we have a Dialog A and S user with assigend SAP_ALL, which user type is the bigger risk?

2. Is it possible to log in with a Service user like with a Dialog A user?

Thank you for your Feedback.

Mike