More useful would be Maths. There are fewer people who really understand Maths, so if you have a lot of potential in that area then go for it.
Plus it's more extreme than Writing. Just like in answers - in Maths it's easy to get 'right' or 'wrong', similar to how most people are either amazing at Maths or crap at it (not saying everybody, just generalising here). Writing ability, any linguistic ability is much harder to categorise. Usually if you fully understand one area of Maths you'll be good at the others too, because they're all logical and if you have a mathematical mind it'll usually work out. Writing is different, I've seen people terrible at spelling and grammer but good at other forms of writing.
More valued by society... it's probably more respected to be say, a mathematician than a journalist. There are less jobs specific to English/writing ability (English is the only subject I really excel in and I'm finding it hard to decide on jobs to aim my life at) whereas with Maths it's easier.
Well, I think a fairer comparison would be maths vs foreign languages, because in this case both tasks would be rather complex. I think that knowing many foreign languages with different roots (so, say, French, German, Russian, Chinese) can be as useful as having a deep comprehension of mathematics. An high writing ability in your mother tongue can be thought about as being akin to good knowledge of univariate calculus, more or less, in terms of complexity.
Yet, being fluent in Romance, Slavic, Chinese and Saxon (not to mention the many others available) would be akin to having a deep knowledge of most fields of mathematics - very few people can reach this level of ability.