You may want to figure out cost of operation when you figure in that your block heaters mostly will be plugged in all night, while you can program the espar to run up to two hours before shift start with the seven day timer, if memory serves me right the espar uses about one pint per hour of operation . The espar is better if you do any runs that may require the bus be parked waiting on a special service group or something where power is not available. The espar does a very good job of keeping the cooling system up to temp in the rear of the bus in winter months.
Your other alternative could be to run airtronic espar heaters which eliminate the need for extended idling, coolant lines, pumps and the water leaks associated with an extra 40 feet of heater hose. We have one bus retrofitted with an airtronic and it puts out enough heat to cook a thanksgiving bird in record time. Without any coolant heat exchangers in the passenger area of that ford cut-away, in winter months the airtronic is enough to heat the whole cabin running on low. That airtronic has two and a half years on it now with no issues.