I went with the 1-acre (40 watt bulb) unit and I am pleased with the results.  After 3 days of use the mosquitoes have been reduced to nearly nothing.  The flies aren't as interested in it as I would have liked, but there may be some fly attractant to put on the unit in place of the mosquito stuff that comes with it; I bought it more for the mosquitoes than the flies, so I am very happy with the results.  I would recommend this to anyone with a mosquito problem.