Lots of quotes about square footage is given as absolute space from one far wall to the other, irregardless of what is between (walls? doors? whatever). Some even go so far as to include a percentage of a public area, like a courtyard.
A better metric that comes to mind off hand would be functional square footage. This is space that can actually be used for things beyond walking from point A to point B. For example, if the front door leads into a 6 foot hallway then all of that space is non-functional. You can't put a table there, a chair, tv, computer, or anything else, without blocking the way. The reason for this metric is that I've seen numerous studio apartments where the layout looks open but the kitchen or a closet is on the opposite side of the room in different corners so to get from the front door to that area a path has to be clear between the two. This essentially lops a side of the room off as non-functional for walking space.
Price per square footage is also a poor metric. I see a lot of places that are charging premium prices for the location or amount of (non-functional) square footage yet have really poor quality otherwise. Many places are getting away with it because of 'premium' location and a lack of good ways to compare quality between places.
Another metric would be quality of apartment or building. My current domicile has very thin walls, a very squeaky floor, no dishwasher, low water pressure, unreliable hot water, old stove, wildly inaccurate oven, degraded tile counters, and a really old style thermostat. All of these combined would generate a terrible score for quality.
Any other ideas for better metrics to compare rentals?