I presume you've already upgraded to the Jet seat piston. That was mandatory awhile back.
A rebuild would include a slightly redesigned connector bolt (both DIN and yoke) to improve oxygen safety. It protects the HP oring from direct exposure to high flow enriched gas. The main spring is now numbered after a few remote failures. I don't know if there was a slight alloy change, or if it was just a bad lot a decade ago. You'd probably get a new piston shaft spring as well.
If it were me, I'd base the decision on whether I'd kept it sealed or not. Several hundred salt water dives with an open ambient chamber means an increased likelihood that salt crystals and floaty coral sand have stuck to the film of lube along the wall of the reg body and scratched the land.
On the other hand, a factory rebuild will likely give you a reg that's good for another 20 years at half the cost of new, assuming the reg body isn't scratched. But sending it to the factory just for annual service won't get you anything a shop doesn't do, unless they toss in some free new components.
But B2 vs M1? It's really just a stainless vs Monel piston, respectively. Meh.
Tough decision without a peek inside.
