There is no need for character Upgrades, it's just a bigger review on the franchise. Lately I see that there is a need to do something of the sort for Digimon because of the differences in the franchise and the meaning of the terms. It is not something that would change the outcome of a fight or something, but the effort to make the text is great.
If I had to vote I'd say inconclusive. Both have enough durability and regen to survive against the other for extended periods of time, but both also have skills that'd let them finish the other in an instant, like sealing or Chrono Breaker assuming these keys have access to that. It's safe to assume neither will open with either of these abilities, but who eventually uses it first would likely determine the victor. We've seen that Alphamon is quite willing to seal something once he's unsure as to whether it's possible for him to easily kill it, as he did with Dexmon, and I'm not sure how often Mille would use his in character so I'm leaning more toward Alphamon, but it's still very possible for either to win.
but both also have skills that'd let them finish the other in an instant, like sealing or Chrono Breaker
I dunno. Because, Alphamon (even in base) transcends time and space. That means he is immune to abilities that affect time or space (duh), that are the only big weapons of Mille. Mille other attacks are pretty meh. A heat laser and Mugendramon's weapons, that are third rate (compared to guys like Chaosdramon).
With his best attacks useless, I wonder if Mille is really up to the task.
I have to ask, tho: which version of Millenniumon we are talking about?
Mmmm... I don't think so. Moon was defeated by a first timer Imperialdramon: FM in Tag Tamers. And tri. outright showed us that the same Impi (now a seasoned warrior) was not a rival for a non-RK Alphamon.
However, if you go up to Zeed, then perhaps this would become unbalanced for a base Alphamon