Is there a weight difference between lugs and fillet brazing? How about using a mix of lugs and brazing?

not much. With fillet brazing you need to reinforce the head tube and you should reinforce the seat tube at the top tube. The bb shells are a little lighter. It's pretty much a wash

+1. The lugs add extra mass, but it takes a fair amount of braze to build the joint up the way people like it with fillets. Silver fillet brazing, which is currently out of favour a little, is both light and low heat, as if the latter mattered for the most part. But it ends up looking like a tigged bike, and there have been allegations of cracking. From a practical perspective, if I didn't want a welded frame (so only so practical....) I would choose lugs unless the geometry dictated otherwise.