I don't find these anecdotal comparisons all that useful since it somewhat assumes no fundamental differences in programs. But the fact is that there are some fundamental differences and knowledge of those differences makes a big difference in how useful the miles can be in each program.

Skymiles awards on DL metal are going to correlate pretty highly with fare prices. Partner metal Skymiles awards to/from US/Canada have been harmonized with DL metal awards, so they will also generally correlate pretty much as well, but they don't qualify for Amex 15% discount. Note that June - mid August is a high demand period, so you can expect both fare pricing and Skymiles mileage pricing will be higher then. Pricing will start trending down in late mid-Aug/early Sept as kids go back to school and demand goes down. Cheapest Skymiles awards will often require roundtrip bookings (pretty much true for all EU bookings, domestically, it varies by market), while FB redemptions are still all one-ways. This is again all about correlating revenue fares with mileage redemption prices. As an example, the cheapest roundtrip Skymiles awards (with Amex discount) on ATL-LAX in Main currently are 27K miles ($357 for revenue fares), while the cheapest when booking as one-ways on ATL-LAX are 23K miles ($294 for reveneue fares).