Greenwood was better last season, Foden has been the better this season. They are both young so aren't the finished article yet.
Greenwood has the better all round ability & is better than Foden. The main question will be if he has the mentality & the club has stability. He is in the goldfish bowl of Old Trafford where he faces intense scrutiny on & off the pitch. The club also have to help in giving him guidance & making the right signings to build the team.
In this respect Foden has an advantage as there is less pressure for him to perform at City. There will also be less intrusion into his personal life. This will give him the freedom to develop. If he has a bad game and/or the team loses, the media will blame the expensive players rather than the academy graduate.