There are 3 major companies producing air conditioners in Israel: Tadiran, Amcor, and Electra/Elco . Other smaller companies Pereg, Nature, etc... also exist, although according to my electrician, almost all the compressors are manufactured by the same company. The big differences is in the finish, efficiency, and features.
That being said, there are differences between the major models in the quietness, and the ability to work in cold weather (under 4 degrees C). Because winters in Israel aren't that severe (even in Jerusalem) you can use many models for both heating and cooling (and it only counts as an air conditioner as far as Customs is concerned). Working on a reverse-pump, it is actually the cheapest way to heat .
Space (the final Frontier): Your first question is, how much space do I want cooled? Remember that air conditioning cannot round corners or go up stairs. You can put in a duct or even a small ventor fan which will move air into an adjoining room. I put a ceiling fan on my staircase and it does bring up the cooler air.
Power: Most technicians will recommend 25000 BTU for a space of 50 sq meters. This also depends on the layout of your apt. If it is a closed room, then it may be overkill. If it is opened with a staircase etc. then you probably need even more.
By the way, there are several models which can be placed on an adjoining wall which can cool two rooms simultaneously. One company is putting out a model which has two sets of fans for different rooms but one compressor outside. There are of course systems for central air conditioning but they are only worthwhile if you have a two-floor house and want most of it covered.
Efficiency quotient: In most countries they give you the quotient which is determined by simply dividing the hp by the wattage used or the BTU by the same. In Israel, some companies give you an Energy Rating. Forget it and use your own.
Split vs Window: A split or mefutzal is where the compressor is on the outside and the fan on the inside. Today more than 70% of all Israeli air conditioners are mefutzal. Simply put, this means that you do not have to make large holes in your apartment, and the noisiest part of the business is not in your living room. However, it is more expensive...
You have the choice in using a floor model or a wall model. The former are usually a bit bigger and there are more models to choose from. The latter has the advantage of not taking away your living room space, as it sits high on the wall. As a matter of fact, most people put theirs over a window so that it is not taking any wall space either. Some salesmen claim that wall models are more efficient in cooling since hot air rises and it is closer to the ceiling. That being said, if you mainly want one for heat you would choose a floor model.
Remote control: This was introduced by Elco some years ago and proved so popular that many other companies have joined them. It allows you the advantage of changing settings without having to go near the unit. If you have a floor model you may feel it is a waste of money, but if it is high on the wall, you really don't have a choice. There is a school of thought of The More Complicated the Better. Chances are something will go wrong.
Automatic Breakers: Most models have an automatic breaker in case of fluctuation of power or a momentary outage. Make sure that it will automatically go back on. If not, then if the outage occurs on Shabbat (and you are Shomer Shabbat) you will get very warm.
How to choose: Aside from making a comparison chart, I advise talking to friends, (obviously who have an a/c unit), visiting them and listening to the noise it does or doesn't make. And last but not least, how does it look? Measure that it fits right . Not just the length and width, but the depth . Will there be enough room to pass around it? Most of all keep your cool