"Just in terms of economy, would a small builder be better served by doing one type of building or the other?"
One sticky wicket is that most Tig builders probably have a torch also, to do braze ons, and drops, etc... So you won't save much on the equipment.
Some of the more versatile builders today will do more than one style, if they figure that will expand their business.
Marketing may dictate what kind of services you can offer. For instance, if you were the president of the local vintage club, maybe lugs would be a natural draw, though Kogswell has emphasized a deeper aspect of vintage, and got away with Asian origin tig welded. If you were a trials rider and all your frinds also, that would tend to push TIG.
TIG makes volume joinery a lot faster. If you had a small shop making enough bikes that you could afford to keep a welding station in full production, then that would be a huge savings in labour over all the work that goes into lugs. But for the guy making just a few frames, or working full time in a one person shop, I don't think it's so clear. You have heavy start up costs, in training and materials. And how do you segment yourself out from the factory? One way is to take it to an entirely different level. To get there is in the 10-20 K range to be all suited up. You don't begin to need to spend than kind of money to do good work, but you might need to put that much into it to convince others you are at the different level.
On the other hand there are a lot of costs to doing the torch work. Good luck with your first OSHA visit, or at least to keeping yourself safe enough while working with all the toxics. There can be quite a lot of cost in tooling like granite surface plates, stocking all the tubes and their related lugs, lugs for various desings, gas fluxing equipment, and cold setting gear to deal with the heat distortion. Stuff for shaping lugs, or getting your own unique lug designs into the mix.
Ask yourself who is going to buy your first frame, as long as you can keep selling them at a realistic price, you should be able to finance the many costly tools that are involved in any of these processes.