I specifically specified "two mages of roughly equal experience and power." Neither of these are balanced. You would need to compare
with
, five lands to five lands. The white mage would be able to cast nearly anything in his deck, especially with an archetypal "White Weenie" deck. The
might be able to cast the majority of his, assuming the cards he put in never needed more than one mana or any one type. So
Counterspell? Out.
Cancel? Out. Now again, if you're talking gameplay, the odds of a 5-color mage getting one of each of the five colors in a row is low, but even then, what if they don't get the
they need? No counterspells. No
? Well, then, no burn. As I've said, the more complex you make your options, the less likely it is you'll have the resources you need when you need them.
You seem to be thinking of this thought experiement in the sense that each said mage could cast any spell, at any time, in the color they have access to. When put in terms like that, yeah, the 5-color mage has a massive advantage, because they can do anything. That's not realistic, even in the realm of an unrealistic concept like magic. And the overchoice phenomenon still applies anyway, because if you have thousands of viable options it becomes difficult for someone to settle on one.
I see. I can understand your argument now and it is a very good one. I shall concede you have point.