Where you will get the most miles depends on the flights you are flying, the fare (including airline-imposed surcharges), and the marketing carrier for the flights (whether the flights start with KL or DL, most likely).

Since both SkyMiles and Flying Blue award miles based on fare+surcharges for own-marketed flights (viz. for SkyMiles DL and for Flying Blue KL) but use a formula based on distance for partner-marketed flights, it's generally the case that a particularly expensive ticket is better credited to the program of the marketing carrier and a cheaper ticket is better credited to a partner program of the marketing carrier.