I see it like that: The Oceanis 37 is probably the best 36/37 cruiser from tha market segment regarding the previous generation (I believe the boat is to be substituted very shortly). Several members have them and they have all posted nice things about the boat particularly in what regards speed and safety in downwind sailing. The boat has also a great interior with lots of storage.
The Jeanneau 379 (don't be mislead it is a 36ft boat, the Oceanis is actually bigger) is a boat from the new generation. You can compare the interior and storage. I never had been inside that one but I suspect I would prefer the Oceanis interior. Regarding sailing the Jeanneau has a performance option (and the Oceanis has not) and regarding the boat with that option I suspect it will make a significant difference to the Oceanis, but then I guess that difference in price will be a lot bigger.
Regarding the standard Jeanneau 379, the Jeanneau is about 200kg heavier, has less tankage (that I think can be increased as an option).
Regarding sailing if we consider the furling
option, the Jeanneau has only 62.2m2 comparing with 67.8 from the Oceanis. With a traditional main the Jeanneau has 70m2 of sail. With a traditional main the performance of both boats should be really close if we take into consideration that the Oceanis is slightly lighter.
Regarding righting moment the Oceanis should be a more powerful boat: The draft and ballast are about the same (with bit advantage to the jeanneau that has a more modern and efficient keel) but the Oceanis has a lot more beam ( 3.79 to 3.92) and that will give it a bigger RM.
The Oceanis will be better downwind and from 60º up but slightly worst in pointing ability and against the wind specially with waves. The rigging
is also better on the Jeanneau in what regards that with the possibility of a self taking jib
that has a better angle.
20K is a lot of money but the Oceanis 37 will devalue a lot when a new model appears and that can be in this year. If you are going to keep the boat for a long time it is another story and I guess you could even find a better price on that Oceanis and you can add a self taking jib
or a small traveler over the cabin for a Jib
It is up to you. Start by the interior and storage and if in doubt, just test sail both boats, the feeling at the wheel is something very important and you can only feel that sailing the boat.
Don't forget also to compare the price of both boats with the same equipment.