I've been using a dynamo setup for the last 3 or 4 years. One of the best decisions I've ever made.
My first hub was a Shimano dh3n72 and I'm now using a Shimano dht780.
Nothing wrong with the 3n72 but it was laced to a 700c rim when I needed new 26" wheels building for a new frameset.
The 3n72 puts out 6v 3w and the t780 puts out 6v 1.5w.
This is where you'll need your thinking cap. If you intend on only running a lamp or 2 then a 1.5w hub will suffice. You'll get a lighter hub with less rolling resistance.
On the other hand, if you find you want to be charging things on the move then you'll need something more powerful. The majority of hubs are 3w.
Regarding rolling resistance/drag, I do find it noticeable when switching the lights on and off. It's negligible.
You'll barely notice the increase in weight tbh
And if you do alot of night cycling it might be worth keeping a small torch/headtorch in the saddlebag etc as dynamo lights are literally bolted to the bike and don't work if the bike isn't moving. I'm thinking of punctures/repairs etc.
Upsides.
I've used battery lights in the past. Lots of them. Very few were cheap. Save for 1 that still runs perfectly, they've all died. Water ingress, wonky circuit boards, failed switches, leaky batteries and in the case of 2 of them, leaping off the handlebars and plummeting to their death. Then there's the cost of endless batteries or recharging. Plus the risk of them being nicked by scumbag Sammy.
The initial cost of my setup was over a hundred quid. Quite a bit really. But, its paid for itself many times over and it never ever fails. I've not worried about run time since fitting them either.
Even at 2.5mph, I get a steady constant usable beam of light.
I wouldn't go back to battery lights if a gun was held to my head.