For me...

Advantages of rack/panniers...gets the weight off me and eliminates the sweaty back and I can carry more weight/bulk comfortably.

Advantages of a back pack...typically less expensive. You can easily take it with you when you get off the bike. You don't have the extra weight and bulk on your bike when you don't need to carry stuff (rec rides). Don't have to worry about heel strike.

I chose the rack, trunk and small pannier option, though.