There is nothing -- absolutely nothing -- that makes those two options exclusive of each other. Tesla can do both with zero obstacles.
An extreme example: If the energy density of the battery were to triple tomorrow for the same absolute cost (ie 3KWh tomorrow costs, weighs, and has volume equivalent to 1 KWh today), Tesla could very easily continue selling a 60 KWh car for a lot less money (and potentially at a much lighter weight if they do some retesting and recertifying), and simultaneously sell a 250 KWh car for a premium price, all based on the same chassis.
The single issue where they'd need to so additional testing and certifying is when they change the mass and balance of the battery... everything else, is just installing a different part into the car, and no more difficult than offering another paint color.
As an aside, there are plenty of people out there who regularly travel by driving four hours in the morning, stopping for lunch, driving four hours in the afternoon, and then stopping for the day. Lots of trips (Miami-Tampa, Miami-Orlando, Miami-Key West, Miami-Cape Canaveral...) need a real-world range of 250 miles at 375 Wh/mile or more. And you want at least a comfortable 50-mile reserve on arrival, which to me says there will be significant demand for a 100 KWh and a 120 KWh battery as soon as Tesla is able to supply them.
I'm not the average guy -- though maybe the average P85 buyer -- but I'd trade in my P85 if they offer a 100 KWh battery on the new ones. That would give me a real-world range of 265 miles at 375 Wh/mile, enabling me to drive the 230 miles between Miami and Orlando without stopping to supercharge and with a 30-mile reserve. Additional battery capacity does still offer significant new levels of flexibility, IMHO... sure there will be diminishing returns from additional capacity, but I think we've still got at least 3-4 capacity upgrades before Tesla needs to worry about there being insufficient demand for more range.