When Mrs Tapir and I were upgrading to smart phones, this was the question I asked, and decided that the only way to find out, was to monitor mobile internet use for a couple of months. Turns out, we both use home and work wifi, so that doesn't come out of our 500mb allowance. Mrs Tapir drives to work, so no mobile internet there, and I use it sporadically on my train commute.
I guess it depends on how heavy you * think * you'll use mobile internet.
500mb is likely to be enough, if you find it's not, then either pay the difference each month, or look for a bolt on.