The air conditioner's BTU rating isn't the only thing you should consider -- the unit's size and weight are important as well. Portable air conditioners are generally between 24 and 36 inches tall, and vary in shape, size and weight. If you intend to move your AC unit from room to room (or bring it up stairs), the weight certainly will be a factor in your buying decision.
All portable air conditioners are not created equal. Depending on how much you want to spend, you can find units with remote controls, programmable timers and built-in air filters. If you are on a budget, you don't need these extras. Most air conditioners come with dehumidifiers or water reservoirs, as well as exhaust hoses and other venting accessories.
Keep in mind that portable air conditioners (as well as wall units) can be noisy. See if you can test drive the unit at your home improvement store to see if you can handle its noise level. As your portable air conditioner operates, water will accumulate and go into its drain tank. With most types of portable air conditioners, this water cools the coils inside and is evaporated (or discharged through the exhaust). Unfortunately, with some units, you need to empty the tank manually a couple times of day. Find out as much as you can about the portable air conditioner you want to buy by reading online reviews and customer testimonials. Ask about the manufacturer (and store) warranty, as well as the return policy before making your purchase.