Due to the window configuration in our rental, portable A/C is pretty much our only option. It hit 115F last summer, so we're not willing to be without A/C. We have both a single hose model and a dual hose model. The dual hose model puts out more BTU's for less energy usage. The dual hose model is also able to send water out the exhaust hose, which is a significant improvement over the single hose model that has a little "pee tube" that we leave in a bucket that has to be emptied periodically. The bucket has overflowed more than once when we weren't paying close enough attention.

If an in-window unit isn't an option then get yourself a Whynter dual hose model. They're expensive, and they take up a lot of floor space (hoses out the back can't make a sharp bend, so expect the unit to sit a few feet away from the wall) but at least they work regardless of your window setup. We've had our single hose model for, oh, probably around 7 years now and I think the dual hose has seen us through 4 summers now.

Make note of the square footage they'll cool. We have the single hose 8,000 BTU model in a bedroom and a 14,000 BTU dual hose model in the main part of our small home. (I'm not sure on square footage but it's probably somewhere around 1,000 sq ft and we often close off the second bedroom to reduce cooling needs.)