What is the anticipated electrical load? Are you going to use a gas or electric water heater? Gas or electric dryer? Gas or electric range? The answers to these questions make a big difference IMO. Without any major electrical appliances, a 100 amp service will rarely see continuous loads in excess of 15-20 amps or so, and usually less. In these cases, the voltage drops over a wire rated for 100 amps will be very negligable.
Even a central AC doesn't take all the much, depending on size. If I remember correctly, the nominal rating is 8 amps per ton (or HP) for a 240 volt compressor. The primary power hogs are the various forms of electrical resistance heating, and if you are using these, then oversizing the wire to reduce voltage drops is a good idea. Personally, I would use aluminum wire for the underground, either direct burial or in conduit due to the much lower cost. This is what most utility companies use, and there is nothing wrong with it in the larger heavier sizes used for services.
