The 2 most important things about the 2 I think are:
– 2 vs 4 decks
AND:
The VCI-380 is the very easy no hassle and no strings attached Serato Controller. Now there is an excellent mapping for VDJ out there, but I think this controller is meant for Serato and it shines there. It is totally awesome for what it does, but that is the end of it. However it is totally easy to get to know this controller and most functions are pretty obvious in what they do.
The VCI-400 is totally awesome with Serato and 4 decks. However it is somehow the jack of all trades. You buy this, if you start out, with a controller that is a lot more complicated and it can get frustrating as you need to really learn how everything works. Not all functions are obvious and you need to memorise a lot of things. If you are up for the challenge it is hard to outgrow the controller and it will be a buddy for a very long time.
Hope that helps.