OK, that's a lie, no one has ever asked me that. But if they did, this is what I'd tell them:
The basic formula for calculating mana is ((Int/5)+2)*level.
What this means is that every 5 extra points of Intelligence is worth one extra point of mana per level, conversely, every extra point of Intelligence is worth one extra point of mana per five levels.
To take an example, let's say you have the choice between a +20 mana ring, or a +3 Intelligence ring, and you're level 20. The bonus mana you'd get from the ring would be (3*20)/5 = 12. So, in this case, it would be better to wear the +20 mana ring. However, at level 40, the bonus to mana from the +3 Int ring would be (3*40)/5 = 24, so by that point, it would be better to wear the +3 Int ring.
Using this formula, you can figure out which items will give you the most extra mana, and at what level you should trade that + mana item in for a + Int item.
Or, short answer, at lower levels + mana items are better, at higher levels + Int items are better.
Now, the exception to the rule, an Int over 200 gives you almost no benefit (it does give you some, but not nearly as much). So, if you have a choice of an item that raises your Int above 200, or an item that gives you more mana, you're probably better off with the mana item, even at high levels.