Spec home wins nine times out of 10. Generally, if you can find a spec home, you can negotiate a price better than a custom builder is going to quote you going into a dirt start. But, if you want to move walls around or you want a floor ripped out ... that likely won't hold.
Golden - like Boulder - is a market unto itself. There may be more spec homes in Golden and that's a good thing for a buyer. I can tell you that it generally holds that a home that is available and sitting on the market is going to price better.
Only the most general of comparisons should be made with square footage. An agent talks about square footage when comparing city to city. We tell buyers what your money will buy in Boulder will buy a lot more square footage in Longmont. A number of factors come into play. But that's a much longer conversation.
And, averages are not going to yield useful information until you have isolated the location where you want to live - and I'm talking about narrowing down your choices to blocks, not miles. Price per square foot is not a good gauge for quality, nor for marketability. A number of factors come into play and the most important one is (drum roll) ... location.
Talk to several Realtors in Golden when you have narrowed down the location you want.