German law states all bikes above certain weight limit must have non-battery powered (front?) lights. It also specifies maximum wattage for front (2,4W) and a bunch of other things (at which speed the dynamo must produce enough juice to run the 2,4W in full). That's the reason for specs.

I once specifically asked a web shop based in Germany to sell me a dynamo light but to replace the bulb with 3W as I'd be using a battery powered rear LED. They declined. The bulbs are readily available, so it's not a big issue to change it yourself.

As far as I know there's no reason why 2,4W dynamo should be automatically better (or worse) than a 3W dynamo. It all depends on how well it's been designed and built.

--J