I was just checking out dynohubs at the LBS last week. The Shimano 3N71 and the SON28 both feel about the same for the friction factor. When the lights are switched off, the friction is negligible. When the lights are switched on, there's a noticeable drag (when turning a wheel by hand) but when you put it on a bike you can barely tell the difference between the on and off resistances.
As for the weight of the hub, here's the tradeoff: An extra pound of hub that generates light whenever you want/need it, versus a one to 1 1/5 pound battery that will hold (on average) only 2 to 3 hours of charge if you remembered to plug it in. You can remove the battery powered lights easily if you don't need them, but you can't have them ready at a moment's notice like a dynohub.
If you're looking to CX with the bike, consider the durability and strength of the dynohub. It's meant for commuting, touring, and long-distance riding like brevets and rando's. Not beating the tar out of it in the mud and on gravel.
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"I feel like my world was classier before I found cyclocross."
- Mandi M.