But which one is really useful?
I've come from programming into a test manager role and can answer this in one word: both.

Though you've chosen a profession now, my 2 cents worth is that both roles are equally important. Software companies need skilled developers in order to build software solutions in the first place, but software testers are needed equally as much in order to spot errors - and there are/will always be some no matter how exceptional the developer is (ok yes I am the 1 exception to this rule and do have perfect code, but I'm excluding myself here).

Career wise from what I can see both professions have specialist and guru areas which can earn the big money, and both professions would probably lead to that status in the same amount of years. I believe development is easier to get started in general. The next step above both professions, the realms of the IT project managers, I'd have to say testing may prepare someone better for as you would tend to write specs, manage people and costs at a slightly earlier time than a developer would.