Backpack
Pros: You already have one
Cons: Back sweat (especially after 6mi)
Rear rack + basket
Pros: Reasonably cheap, and you can just throw stuff in there w/out thinking about it. When your lunch leaks/spills, clean up is as easy as going for a ride in the rain.
Cons: Not waterproof (will require a plastic bag if riding in the rain), and stuff will fall through holes if it's not mesh-lined.
Recommendations: Axiom Journey is my favorite rear rack; it can handle lots of weight and runs around $40. Compare that to other quality racks like Tubus, which are $100+. Zip tie a milk crate to it for a cheap option, or get Sunlite Mesh Folding Baskets for $25 each (just be sure to reinforce them with zipties, as the folding baskets aren't as robust as the rack).
Rear rack + waterproof pannier
Pros: Waterproof. You can find combination backpack/panniers for extra convenience.
Cons: Expensive. Loose cables/straps or not attaching the pannier well enough can get things caught in your rear spokes, damaging the spokes and/or pannier (and/or your face when your back when suddenly locks up).
Recommendations: Ortliebs waterproof panniers are $150 each, but well worth it. One of the things I like about the aforementioned Axiom Journey rack is the wide side; too many other racks look like a triangle, which can lead to the corner of your pannier hitting your spokes. A rack with a wide side will protect against that
Front rack + basket/large platform
Pros: Same benefits as rear rack + basket, plus you can see what your carrying at all times. You can probably reach in and snack or drink while riding. Probably cheap.
Cons: Severely limited in weight. A rear rack can take 50+ lbs with no problem. A front rack (attached to the fork) gets kinda hard to steer at around a mere 25 lbs.
Recommendations: Origin8 Classique can be had for $60, and has a platform wide enough to carry a pizza. If your front wheel has a quick-release (instead of a bolt), Sunlite QR-Tec can be used and is only $45 (but carrying a pizza is more iffy, and the installation is painful). Pair with a folding basket or milk crate.
I wouldn't stick panniers on a front rack. I know people who do, but I've had my rear wheel lock up before. It's unpleasant but I've managed to keep control of the bike. Locking up the front wheel is a different matter entirely; you fly over the handlebars immediately. For that reason, I prefer things that attach solidly to the front rather than loose bags.