Experienced players already love Chaos mode, and if you're thinking of diving in, well, it's a very different experience. Chaos mode is how the game was meant to be played. Just because you can summon a monster doesn't mean it will happen. Just because an attack did high damage previously, doesn't mean it will this time. Chaos adds a random element to a lot of the game's mechanics, and while the randomness of summons can be offset by pick ups, there is never a guarantee.
Improvisation
The true benefit of Chaos mode is truly fighting against the odds. You don't know if you can cast spells or summon monsters, do damage, and yet you have to fight against the tide. The spell cards you draw might not be what you want, and on top of that, you have to deal with the fact they might not all work. Though that certainly doesn't mean things are impossible. Improvise your way through the struggle, and you'll come out feeling more rewarded than possible in Law mode.
Law and order
Law mode is most certainly where you'll want to start if you're new to the game - this tactical adventure is pretty punishing if you're not careful. Law mode removes an element of randomness from the game. You will always be able to summon monsters, given enough mana, and you'll deal set amount of damage from moves. There's several other small changes between the two modes, too, all of them making Law a more comfortable play experience for beginners.
The thoughtful play
It's for the reason of randomisation, that the Law Mode will be the preferred game mode of traditional tactics fans. A majority of other games give you guaranteed results from your moves - like, well, chess I guess - and Law mode is there to accommodate those players. Though, it does also mean that things will be an awful lot easier, so if you're feeling confident, give certain missions another go in Chaos mode.
Complete chaos...