The H400 is essentially an H100 with a larger detector volume. In the same scenario, an H400 will provide results 3 times faster than an H100 because it has more CZT.
A more subtle difference between the two is that the H400 is more sensitive in the forward direction compared to the side directions. This makes the H100 better in situations where a uniform sensitivity is important.
An additional H series option is the H420. The H420 is very similar to the H400, except it is able to image low-energy gamma rays, down to 50 keV.
If you have more questions related to the H100 vs. H400 ask them here!