Suppose it depends on what you want as the a car with 20d or a 35d engine is a completely different beast and providing different levels of running costs. If it were me, and it was possible, I'd go with a slightly older 35d with all the toys.
Don't forget that a 6pot car will come with a higher level of spec than a 4pot as standard so depends what options you're interested in.
Current -: 370Z GT Roadster / Qashqai+2 Tekna 1.6dci
Gone -: BMW X3 3.0d xdrive M Sport; E46 330i M Sport Coupe; Mazda RX8 231; Nissan 350Z GT Roadster; BMW E90 330d M Sport; BMW E92 335i SE; Maserati 4200; Nissan 350Z; Honda S2000; Astra Coupe Turbo; Ford Probe