Mexican tanks are a bad analogy. If anything the ocean is a better analog for space than national borders. Foreign warships routinely pass through stretches of ocean claimed by various countries.
The diplomacy win in Twilight is probably easier, but that's a game strength. One thing that's done better than in Civ4 are computer players reacting to past behavior. If you get a couple of the minus modifiers for going to war or conquering others you can lock yourself out of it. The reason Terrans have an easy trip to diplomatic victory isn't just the super diplomat ability, it's that having the ability forces you to play nice and take actions the AI smiles on.
I'll grant you that it's likely faster, on average, to secure a diplomacy victory with the Terrans than a conquest victory with the Korath. But it's worth bearing in mind what happens when the positions are reversed too. Should your continuous sporing slow down, you can solve that by judicious use of Operation Big Hammer. You build up an even larger military until they beg for mercy. The Terrans, on the other hand, have to research and then build a military if diplomacy fails. So while one is faster when it works, the other is faster when it doesn't.
The worst part to diplomacy wins in my book is the fact that they're entirely too easy to finalise in a week. Once I've opted to take that road I'll wait until I have close relations with everyone else, negotiate peace accords between all warring civs (perhaps throwing in the alliance tech to sweeten the deal), form alliances with everyone, then hit return. The problem isn't so much that it's easy reaching close with everyone, as much as the fact that you can completely eliminate the risks of forming alliances.
