Eh they are both different birds.
The DA40 flies far and fast and is well equipped (trim switches on the yoke, rather well designed cockpit), it's IFR rated and it's got a good A/P. The engine is amazing too. The only drawback I think of is the seats (you can not adjust their position and they're not very comfortable, especially during long trips).
The Diamond is also a real pig to land. Idle too early and you're landing quite firmly, idle too late and you won't be able to land it.
Meanwhile the C150 offers a cooler ride, you don't have much things to look after. The cockpit is well designed and offers a great forward visibility which is interesting when searching for traffics. The full anagogic cockpit looks great to me. And last but not least, our long range C150 has two huge 72L tanks which allow it to fly for 6H (roughly 1100km). As everyone knows, the C150 is quite easy to fly and is quite forgiving. Smooth landings are easy. Obviously the 150 is an old bird and just cannot compete with the DA40 performances. It climbs slowly, flies slowly and the seats are rather uncomfortable if you don't take a cushion.
So I couldn't say which one is my favorite, it depends whether I'm planning to do a flight during which I expect a heavy workload (DA40) or a cool flight during which I want to actually fly the aircraft and get back to basics.