I don't know where you are at. If you have wet field conditions then the JD 6620 will go better than a IH 1440. The rotor machines are heavier in the back. Red or green.
As for harvesting. In high moisture corn the IH will do a better job. In wheat or green stem soybeans then the JD 6620 will do better.
As for parts. The JD 6620 will have many more options for parts. There are many after market companies that make a lot of parts for them. The IH has after market parts too but not near as many options.
Then you need to consider what heads you are going to use. The IH corn heads and grains tables are not near as good as the JD ones that would be on a JD 6620. IH did not have a very good grain table until the IH 1020. The IH 810 was real bad and the IH 820 was not a whole lot better. While the 200 series on the JD where a real good head even yet today. An the 40 series corn heads where just bullet proof.