2 Smaller Window Units (2 x 12000 BTUS w/ 10.7 EER) or 1 Larger Unit (1 X 25000 w/ 9.4 EER)?

We are looking at cooling a house we bought a couple of months back that we have been fixing up. It was a foreclosure so ofcourse the heat pump was missing from the house. We will eventually have to shell out the money for a heat pump but want to prolong it until we need heat (which is SC will be beginning of October) to give us a chance to save up for it. I am looking at window units to get us by for the summer. House is 1.5 stories, 1900 sq ft - 400 of which are upstairs and we have an 8000 or 10000 btu air conditioner we bought a year ago that can cool that when we are upstairs. With the 1500 or less sq ft down stairs I am wondering which will be the better options as far as our electric bill goes. I mentioned two different options, two mid size units, or 1 big unit. The pros of the smaller units are they seem energy efficient and can run off 115 volts so i wouldn't have to change anything electrical, also they can give me more control (say i wanted a bedroom to be cooler at night, etc.) However my main concern is my energy bill. Will the larger unit be more energy efficient even though it has a lower EER number because it is running off 220v and everything is coming out of one location? Need someone with more knowledge about this to help. Thanks in advance.