If we’re talking ‘real’ Civil War, Syria is our guide. Predominantly wealthy, educated secular cities siding with the government against predominantly rural and poor. The military split very early in the war which is a possible outcome. In modern civil wars, land is not particularly valuable. Open spaces are hard to defend from air power. Cities are much easier to defend even against modern adversaries. Block to block fighting is deadly. Resources aren’t really as much of an issue. Mountainous areas are really the worst places to attack and in most civil wars we can see that battle lines tend to die there with very little movement. Little villages like Kinsibba took months to take of back and forth fighting.
It can be assumed that there are plenty of powers that will be friendly to both sides, so we would expect arms to make their way into the country from those places. There is likely to be a lot of asymmetric warfare and fighters fading in and out of the populace. What we would see happening is that largely most city infrastructure will cease to exist regardless of who wins. My guess would be that in a true civil war scenario, the large portions of the majority of US cities would be practically rubble. If we look at Syria again as a guide, no part of the country is untouched. With Latakia being the only part that is relatively OK and it came within a few months of being annihilated. In contrast to the American Civil War where battle lines were fairly static and troops relatively isolated, assume that everywhere would be a potential war zone.
So, the red state advantage is that the battles likely won’t be fought in red states and would be fleeting affairs. People in the countryside have more room to run, but also are more open to human rights abuses since they are more isolated. Blue state advantage is that cities are defensible. Overall though, modern civil wars are not good things. What we see as the new trend is that actual fighters die at far lower numbers than civilians. We have learned that taking cover in civilian populations is a way to win and no state has yet been able to figure out how to deal with the situation other than kill them all and let God sort it out.