On the SBR, it comes down to paperwork.
If you file a form 1 (tax paid manufacture) to SBR it, you are in possession until the form 1 is approved. If not approved, you retain your parts. You are the manufacturer, and it's your info stamped on the gun. You send to your builder (ANY FFL) and receive directly back.
If you send the kit to an out-of-state builder to register, they must be a Type 7 & SOT. They file a form 2 (tax exempt manufacture) and include it in their annual report. On completion, they ship it to a dealer in your state on a form 3 (tax exempt transfer). Then you file a form 4 (tax paid transfer) which is essentially the same as a form 1, and the dealer has to sit on your gun until your form 4 is approved. If it's not approved, you're stuck.
I don't like the second path, because it's more paperwork for me, but not less paperwork for you. And you have to do a 4473 to pick it up. There's no upside to it. It is for buying an already-built SBR.