I think the question idea is better for preventing spam. The captcha requires a human to type in the string but that just means you need a real person to type it.
Wish I had a great idea for a question, but that has the advantage of requiring someone to read and respond in English (and since this forum is in English, that doesn't seem unfairly restrictive).
Actually, I think I'd make it less simple but more intuitive for English speakers, if that were possible, but could be hard since you have people from so many countries who speak a slightly different English. That's the problem ... Everything harder I can think of is probably not commonly answered the same way in England or Australia, etc.