About Unit overlod of the map:
Practical solutions to unit overload could be two-fold:
1) In the same way that a City can support only so many people (ie. its population rate), you could put a limit on how many units a given city could support. I'd make this a hard-cap (as opposed to a soft-cap that only penalizes you for excess units) because:
- Sometimes penalties, whatever their nature, is not enough of a deterrent;
- A hard-cap is easier for the AI to deal with.
2) Most of the time, one has a flood of units because he keeps outdated units from bygone eras. The AI is especially guilty of this. A solution would be to auto-promote units to the current equivalent. I'd make this available not immediately upon discovering the new tech, but maybe after a certain number of turns has passed, or some other trigger; thus allowing the strategic decision of paying to get the upgrade faster.
I think if your archers turned into crossbowmen after, say 15 turns (just an example) of getting Machinery, the urge you'd feel about building units
ad infinitum would be lessened.
About 1UPT:
In an ideal world, a middle-ground between 1UPT and SOD would be programmed into the game. But, as was written before, the upkeep-per-tile idea is more complicated than some may think.
It IS complicated for human players, because you'd have to carefully check at the end of each movement if the tile is able to sustain your unit; that's more micro-management, and not of the good kind.
It would be NIGHTMARISHLY complex for the AI. You think the AI has difficulties dealing with water-tiles? Believe me, you ain't seen nothing yet. An upkeep would literally paralyze the AI. I'm willing to bet that it would start loosing (or completely crippling) units, in turn urging the AI to build more units to compensate for the loss in military might. The AI, who already builds way more units than needed if used properly, would now build nothing else, making it impossible for it to win in any other way than with a military victory (and we already know what the odds of that are, right?)
All of this to say that I'm on the side of the extremists

and believe the only practical options, from a game design perspective as well as gameplay, is to go with one or the other - 1UPT or SOD. Personnally, I think 1UPT is more interesting (even with all of its faults), but that's just my opinion.
A word finally about those that argue that 1 unit occupying the same space as a city: it's not. The city is not only the single tile where it was founded, it's all the tiles around it where people of that city are working to produce food, etc.
Furthermore, when I read some comments, I am under the impression that those people think that a unit represents a single person. It's not. It is as many of that particular unit type as are necessary to have an impact on the battlefield while having a manageable number of people. That number fluctuates depending on the particular type of unit. An archer unit might be 100 archers, while a Tank unit might represent 10 tanks (the numbers are just used to illustrate my point, please don't argue the numbers themselves). So is 100 archers in a tile realist? Not completely (the game is an abstraction, after all), but it is not that far of a stretch.