Personally I have never had much trouble booking on either DL or NW. Both tend to make empty seats available for award redemption at the last minute (2-3 days out) but DL charges you a $75 fee for booking Last Minute trips. If you are looking at strictly RT than they both work about the same, however, on open jaws DL has a more liberal policy. On NW the open Jaw has to be smaller than the flight segments. For instance if you are going from DFW-PSP and driving cross country then flying back RDU-DFW you would be allowed to book that as an award on DL but not on NW. So if only getting one flight is your goal I would probably say DL since you will earn 20% more miles. But also remember that the two programs are code shared. So you can actually earn the miles in a DL account and then use those miles to book flights on NW metal (as well as CO and any other Skyteam partner flight).