well u can see my view of them in my earlier post to bernhard.
battle magic gives better result early in the game as most spells have relatively low mana consumption. plus, their damage-to-mana ratio is better than nature magic, especially obvious in the early parts of the game. another advantage is that it has a lot of destructive area effect spells.
nature is relatively weak when u start developing it. but it gets better & better as u progress & it has quite a few(IIRC) very powerful spells that u unleash on a single enemy. this is perfect for bosses. plus, it has healing spells which a few are area effect. this is great for party.
in this game, it's best to have each character specialised as the reward will be more obvious. each will be the best in his/her field. u don't want a bunch of half-baked disappointments. unless u want to go solo.