I would suggest, in addition to the 3 quotes that you are providing, that you contact (or suggest to the GC to contact) about extending distribution voltage underground to a pad mount transformer near the home. In many areas the customer provides things like trenching, conduit, a pad for the transformer, etc. These are all built to the POCO specs.
The benefit is that the 600 foot run is all done at high voltage, greatly reducing voltage drop. Also any continuing energy lost in the 600 foot run is _before_ the meter, rather than having the customer pay for it. The POCO is in charge of the high voltage portion of things, so you don't have to worry about installation issues that you are not familiar with.
Some other points:
The reason to even consider the transformer arrangement is that it permits the use of smaller wire to get an acceptable voltage drop. This is the exact reason that the power company distributes at higher voltage and then uses transformers to step down to the usage voltage near the loads.
Consider delivering 24KW at 240V and 100A to a load. If you had some way to operate at 480V, then you would only need 50A to deliver the same total power. Half the current means half the voltage drop in a given conductor, but it gets better: the voltage drop in volts is halved, but in % it is quartered.
In your case, with your 240V supply and minimum sized service conductors over 600 feet, you are looking at perhaps a 10% voltage drop if the full 200A is used. If you could somehow use 480V to deliver the same power, then with the same conductors you would only have a 2.5% voltage drop. The higher the voltage, the better things look in this simple view.
Unfortunately you have a few complications.
1) What current should be used for the voltage drop calculations? A residence with a 200A panel just doesn't draw 200A on a regular basis. But if there are some large motors (well pump, hvac, etc) they could draw much more than 200A for very short periods of time.
2) If you use two transformers for step up and step down, in addition to the voltage drop in the wires you will have voltage drop in the transformers themselves.
3) If you use the transformers then they are running 24x7 consuming power because transformers always have magnetizing losses. Even if the transformer approach is the least expensive up front, it will cost in terms of long term power consumption.
4) As others have noted, you are limited in the voltage that you can reasonably use.
-Jon
