One of the things I like about MageWars is that there are two stages at which you're being competitive: the build of the book and the play of the game.
This means that you can be flavourful with the book, but still 'play to win' in the game itself. I prefer games when both people are playing to win, as that's how the games are built to work, but it works fine for me if the books are built on a different basis, as they're a set quantity once the game starts.
As well as allowing flavour, this means if I'm playing an opponent I usually beat I can get a really close game without pulling punches at all by, for instance, playing an army-building warlord or a ranged-attack Johtari beastmaster