You have to consider your climate, the house layout (are the bedrooms all up stairs so at night you could cool them without cooling the whole house), the condition of your windows and insulation, and how much longer you will be in that house, as the fancier multi units with variable speed (so they can run slower to better maintain a consistent temp, keep the humidity even, and use less total power since the start up is the most power hungry part). The higher the SEER the, in general, less energy it will use but if its a 10-15K difference in installation price for $100/mo savings, that's a 10 year break even ROI, if you plan on moving in 3 years you won't see that savings.
For example, in my house I have a upper story and lower story unit. For cooling they both run since hot air downstairs will just go upstairs anyway. But for heating I can set the upstairs to be a bit warmer at night since no one is downstairs. Programmable units can get even slicker to just cool specific rooms but thats a lot of investment in ductwork. At the time I got my units, the windows were old and leaky, so I just got single stage units since where I like it is nuclear hot for 6 months a year, the A/C is basically on all the time anyway and with leaky windows I wouldn't see much advantage to the variable speed. But now that I've replaced my windows, I kinda wish I went with the VS just to see if I could go all day without needing the A/C on full blast. Oh well, you can't go back, thats for sure.
Also, if you have the old refrigerant (R-27 or whatever), that stuff is now SUPER expensive versus the new R-117, if you are replacing your units anyway, you might be able to work out a deal to sell the old refrigerant separately since the A/C company will just take it and then sell it themselves otherwise.