Obs you can get lots of BPs for just by chasing them in events and that goes for any age
Arc you have to either be in that age and above or contribute to other Arcs
It's just more likely you'll have a obs then a arc unless you're chasing after a arc given the obs takes less space and the game gives plenty of opportunities for players to get the obs
Both give the guild goods which is what the guild is really after
Not to mention anyone can get Bronze Age goods to build a obs. Not everyone can get arc goods