For each SEER that the unit rating increases there is an 8-10% savings on the cost of running the air conditioner unit (it will use that much LESS energy).
The highest rating of 21 is best suited for all year round in hotter climates, for example Florida, as the air conditioner unit will potentially run all year and you will want the most savings. These air conditioner units typically cost the most.
For Southern Ontario, this including Brantford & Cambridge, the most popular air conditioner unit has a SEER of 15. This would give you good quality and efficiency for your dollars spent.