To irrigate or mine?

On grasslands, plains, and deserts, how do you decide whether to mine or irrigate? Personally, unless a city has very little food (surrounded by mountains and hills), I pretty much always put a mine. My rational is that I'd rather have a smaller but more productive population (within reason) than a larger one, since larger populations mean I have to deal with happiness issues. I might be missing out on income from that higher population, but then again I'd have to use that income on happiness. The AI seems to almost always favor irrigating. How do you go about figuring out whether to mine or irrigate? Are there any good guides out there?