I have shot both ways, but I prefer to shoot directly off the string. The advantages of a loop are many, no serving wear, helps in keeping peep lined up, no nock pinch, etc.
I have most of mybows set up with a loop anymore, but I still set one up without every now and then.
Aproperly tied in D loop is nice but don't expect any earth shattering changes in shooting accuracy, etc. It really boils down to personal preference as both will work effeciently, but a loop has advantages that can be nice to many archers.
Dan