Thanks All! I think it is clear to me that for my travel pattern (couple of international business class round trip tickets and couple of domestic economy US tickets)
1. I could reach FB gold (100 + 180 XP) or SkyMiles silver. (5K MQD)
2. It is easier to maintain FB gold. (I just need 180 XP per year once I have reached gold status) (Of course by the time I have the system working as I would want, the point system/rules will change).
3. FB gold is better than SkyMiles silver for almost anything that I want (Being able to select a seat, sky priority, even potentially upgradability?)
I have already booked my tickets on delta.com. I cannot quickly figure out how I ask delta to use my FB number instead! (I also have to yet create an FB account, Are there any promos for creating an account?)
ETA : Found out how to do that. Under my trips, select the trip, then go to the dropdown of show more, that adds a new option - "Loyalty and Benefits" I can add FB number under that