Should Modis & LandSAT NDVI composites agree and why not?
I can't comment yet, but I want to share this thought:

One challenge of the comparison is the co-registration of the data, so that really the same area on Earth's surface is compared. 

In this article about the difference of MODIS and Landsat 7 NDVI, the authours put much effort in the co-registration to really compare the same area on the ground (but still there was a difference between the two sensors)