Bottle dynamo vs. hub dynamo
I have not used a bottle dynamo for a very long time, but have a hub dynamo. Broadly, the hub dynamos are more expensive but more efficient - you get less drag for the same power output. Most systems put out 3W peak power for historical reasons, although a lot of new lights use less than this. Bottle dynamos are noisier and wear on your tyre, which may be an issue if your tyres otherwise last a long time. A good bottle dyno will be nearly as efficient as a hub dyno and cost about half as much, plus you don't have to build it into the wheel. A cheap bottle dyno should be removed and thrown away.

Some dynamos will put out more than 6V, which is how you can get more power out of them (the Schmidt ones do - I used to run a 5W white LED straight off one). Often at decent speed they will go to 12V at the same 500mA, so you can get 6W. There are also dynamos specifically designed to do this, and they usually come with a regulator and various output options for charging things like phones.

Hub dynamo systems cost at least a couple of hundred dollars because you need to build the dynamo into a wheel. But they last a long time (I don't know how long as I've only had mine for 10 years and the ones I know of that have failed have done so when the bike was destroyed around them, or the owner pulled them apart). Any decent LBS should be able to get them if they don't stock them, or refer you to one who does. Peter White Cycles is one of the better dealers in the US with a wide range of options, likewise St Kilda Cycles in Australia. 

I think this is a reasonable overview and this gives more details, specifically discussion of the "more power" idea.