At least for my US-based AmEx, Flying Blue and Delta transfer at the same rate. But it's easier to find cheaper FB redemptions under normal circumstances (both Delta and FB have their share of redemption sales) Plus for Delta is that miles don't expire under current program rules.

Delta and Virgin currently have a good partnership outside of Sky Team. Virgin has also recently formalized a codeshare agreement with Air France/KLM and is scheduled to be fully included with the Delta/Air France-KLM joint venture at some point in the next year. (If you're going to be hitting secondary US cities, Delta also tends to have a more stable product with better IRROPS recovery than American or United domestically right now.)