I bought the K40 to save on space while stowing it on my 13' boat.
It's not as fast as a barrel pump to fully inflate a boat, but I don't do that very often so I don't mind.
I do use the top-up function sometimes, but only on the floor... it doesn't seem necessary to us this feature to get the main tubes up to pressure. I'm very happy with the purchase.
I'm sure you can't go wrong with either, and if you have room for the K400 it couldn't hurt to have more volume.