They are all pretty ineffecient. Better effeciency can be had by spending more money on central A/C (higher SEER) or a mini-split (for example Fujitsu's run up to a whopping 21 SEER). In both of those cases you would need a professional install.
That being said, a dinky little A/C like that, even running at 8 SEER or something crappy like that, is not going to break the bank just because it's not big anyhow, and you probably don't run it 24/7. Additionally, depending on your situation, wasting some electricity dollars may be ok if the alternatives (putting central A/C in your rental unit or something) don't make sense. This "ineffeciency" may add up to something like $10 a month (guessing here) so it may not matter.
Jensington