The appealing parts of SkyMiles are targeted towards the domestic/US based flyer. Things like upgrades, priority security etc. Delta's award redemptions can be tough. And redeeming from abroad is expensive with Delta's international origination surcharge. One of the few pluses plus I could see would be access to Economy Comfort if you make it to PM or DM. Another is lounge access at GM.
The IOS is the YQ, which is the same as charged by FB, so there's no disadvantage there. In fact, since the IOS is charged almost exclusively on European originations (plus ex-TLV on AF/KL metal, apparently) and tickets where DL collects the partner's YQ (in which case origin doesn't matter), if OP would be booking award tickets originating outside of Europe, DL is the better program since FB collects YQ on all redemptions.
If I recall correctly, FB Plats get EC for free, and Gold and Plat with FB are ST E+, so no real difference there.
I'd probably analyze the paths to status (including segment-based) in the two programs and see which one provides the easier path to obtaining the status level you desire.