Originally Posted by MSPeconomist

For DMs, the opportunity cost is a bit over 4000 miles per SWU, at terms of trade of 6 SWUs or 25,000 bonus redeemable miles.

A big difference is that SWU upgrades can happen at the gate, while miles upgrades officially must be confirmed before e day of travel, but YMMV. In fact SWU upgrades if coded right have very high priority at the gate.