Your choice of future FFP really depends on your booking patterns and preferred redemptions. I'm also PRG based and very happy with Delta as primary FFP. But it's because I'm almost always booked into discounted economy fares and redeem outside of Europe.
It might be easier to get status on FB but you earn very few miles unless you book higher fares. FB is way better if you want to redeem for flights within Europe since they no longer charge YQ on awards and run discounted award promotions.
DL on the other hand is useless for redemption within Europe as you often end up paying less with a revenue ticket. On the plus side you almost always get full miles even in coach. If you want to avoid the hefty YQ ("international origination charge"), your choices from Europe are extremely limited to KE, AM and Saudia. You best use your miles on awards for trips originating outside Europe. Plus all the usual downsides of SkyMiles that are discussed on DL forum apply. DL still may be a great choice for some but not all euro flyers.