The Build quality is the same. Materials are the same. Killer-b may spool earlier. These ones will breath a little better due to longer primaries.
These also can came with inconel flex section and the price is about $400-700 less depending on setup. I don't see the holy headers being worth the extra 4-7. These are both top tier parts. Just like ETS, killer B and the rest of the 421ss crowd it comes down to what u want.
Bellows is just another term for a flexible section. Inconel bellows are about $350 a piece so they are not cheap to add.