I've tried all three and found that it really depends on what you're looking to carry.
1. Backpack - Team Quattro Kawasaki bag I picked up at a British Superbike round. It holds a fair amount, but it does begin to get irritating on longer journeys. The straps can be a pain, but it's more the fact that the pressure in this position will make your back ache. Given that it was cheap, I'm not going to count it lasting forever either. I'm ex-Army and am more than a little familiar with carrying heavy packs.
2. Oxford Lifetime X30 tankbag - First piece of proper luggage I ever got. It expands to a decent size, the magnets attach in seconds, everything is right in front of you for easy access and the clocks are still visible even when filled. Not the cheapest item, but when Oxford say 'Lifetime', it appears to be a pretty accurate description. Shoulder straps are included which makes carrying it around less of a chore.
3. Oxford 50 Rolltop seat bag - This was a gift for Christmas, and not yet used in anger, but everything inside stayed dry on the 20 mile trek home in moderate rainfall. I'll be taking this with me on my [still in planning stages] trip from the UK through France to Madrid this summer. Plenty of capacity, but probably overkill for day to day unless you pack the kitchen sink and extra spare taps too.
If I had to pick one, it would probably be the tank bag. Holds a basket of groceries when expanded, doesn't give me a back ache, and even without the included interior waterproof sack, I've never had so much as a drop of water get inside.
The best advice I ever received was "Just look pretty and try not to break anything."
1999 ZX6R (G2) - Pipewerx exhaust (dB killer removed) | Dynojet Stage 1 | K&N air filter | flushmount front LED indicators | Clear LED tail light with integrated indicators | Scottoiler eSystem | Pyramid Plastics hugger | HID headlight | Stubby levers | HEL braided steel lines | 07 ZX6R radial master cylinder | Bar-end mirrors | Double-bubble screen | Crash bobbins | one hell of an anal-retentive owner.