Drawbacks of a bottle dynamo:
- It's a significant noticeable drain on your speed. It works by rubbing a piece on the tire, it eats up noticeable power. Hub dynamo's are far more efficient and don't do that.
- They can be noisy. Again because they're physically rubbing against the tire.
- Because a bottle dynamo has an external part exposted to the elements, it's far more affected by rain, snow, etc. I don't have one so I can't say I've only heard that they can start slipping and not working in the rain (perhaps newer models don't have this problem), but if your winter riding involves snow or freezing rain I would think there's a good chance it would wear out the roller part pretty quickly, suddenly your light wouldn't work in the middle of the ride. In contrast, a hub dynamo is completely sealed and not exposed to the elements at all. My winter bike for Minnesota has a hub dynamo.

I can see how a bottle dynamo could be fine for a fair weather rider. Or someone who wants a no-maintenance backup light.

But in my opinion is that advantage of a dymamo system is it's "no maintence" approach so I would only go with a hub dynamo.You always have a working light with you. No "shoot I forgot my light at home". No "I forgot to charge it". No "I charged it but my ride went longer than I thought and it's out of juice". No "I didn't realize the battery was running out of capacity and it turned off sooner than I expected". It's like your car headlights where if the bike/car can move, the headlights work. (Actually it's more reliable as most car headlights are halogen which burn out whereas dynamo lights are led's and do not not burn out).

But they're also really expensive. When my brother wanted a light, I bought him a battery light because I didn't want to spend a huge amount of money on it. With a usb charger cable in the garage he can charge the light while it's on the bike and he doesn't forget it. It also works well.