Hey Steven. The entitlement process is tricky most the time. Even when we try to simplify by going through County versus City, there are still nuances that you'll want to make sure you cover before you move on your decision. I'll put out the disclaimer that I do land development and entitlement in Arizona mostly, and more specifically to the PHX metro area, so I'm not as familiar with Colorado.
That all being said, here in AZ, most the cities I work in are also in Maricopa County. If it's a County island, there is an understanding between the Cities and the County that they will work hand in hand. Some items to be aware of include, but not limited to: zoning in County may conflict with the City, utilities that belong to the City infrastructure may require that you annex the property prior to rezoning or other entitlement, County development standards may be different or less stringent than City standards which would cause the development to be non-compliant when annexed.
I would echo the same as Bryant Brislin, you will definitely want to connect with someone that can navigate this process for you. Small obstacles can become very expensive or time-consuming when you don't know the processes.
I hope this helps. Good luck!