Off the top of my head there are 3 or 4 differences between the two,
the nozzle is Longer, which requires a aluminum block to go in front of the mech box and screws in to the lower receiver, the lower receiver has a wider mag well to allow the bigger mags, which are proprietary and only VFC mags will work. (my roommate and I have successfully put a Magpul 20LR mag modded of course in the VFC H and working) the spring loaded battery terminals in the lower require a metal spacer between the gear box and plastic cover, other then that, the gear box, hopup, and upper receiver are the same as well as the stock.
It really comes down to personal preference, the L is more versatile as it will take any m4 mag, where with the H your limited to VFC mags. In airsoft there is no real difference besides the mag for performance, In real steal u have your L which is 5.56 or your H which is a best firing 7.62 rounds.