at the rates you quoted: Propane = $3.00/gal & 1 KWH = $.05 then electricity @100% eff = 68,260 btu/$1.00
Propane @ 85% eff = 25,953 btu/$1.00

Additional things you need to take into consideratrion are first cost, replacement cost, cost of capital, and maintenance. Without doing the life cycle cost analysis, My take is propane is cheeper than electric.