Generally, most residential forced air cooling applications use the high speed setting, and heating applications use low setting, and they switch between the two according to the call for heat or for cooling. There are a couple of reasons for this: exchange efficiency, cooling efficiency, and human comfort (ref ASHRAE for details).
Air moving at 15 FPM cools people better than slower moving air. Registers and fans are sized, and types of registers picked, to get that flow in the cooled room. Air moving much over 5 fpm in the cooler months will feel drafty if the air is below about 65 F.
Cooled air is heavier, and there tends to be stratification (cold air pools at the floor) in rooms with inadequate flow or improper pathways. Higher velocity air tends to fix a lot of problems with stratification.
Heat exchangers transfer heat to or from the exchanger into the air. First, cool air is heavier than warm air, and needs a bit more pressure to pass properly than warm air. Second, cool air will deposit its moisture all along the exchanger depth if it moves slowly. A properly sized and designed exchanger/system will drop the temperature below the dew point, extract moisture, and raise the temperature back up so the air is cool but not water-logged.
So basically, the airflow in the room cooling and heating determines the fan setting, and the flow across the exchanger efficiency is checked by determining pressure drop across the exchanger. If the pressure drop or temperature range is off, (see the table in the installation manual), airflow is changed to get them within range, pressure first. If they won't both get in the range, get the pressure right first and then adjust dampers, open/change registers, etc., to lower pressure To check for temperature drop, get a temp probe at your home store or auto store - about $5, maybe $10 max. measure above and below the exchanger and compare to the table (on one side of the airflow to the furnace/cooling coil, they just poke thru the fabric sleeve on bonnet - don't poke the exchanger ! The other side usually has a hole or plug to take out for access.) Pressure drop requires a manometer for reading inches of water, available for around $ 150-500 from a supply house, or you can make one for about $ 5. Buy some 1/4" clear flexible tube that can reach from about a foot above and a foot below the exchanger to a piece of board, with about a foot or two extra. One end goes in at the top right corner of the board and down the side of the board, and then angles back up from the bottom to the upper left corner. Mark the board in inch marks vertically, and put some water in the tube. How much depends on what kind of pressure you are looking at - 6" of water means you need a ten inch tall board and about 7 inches of water in the tube. Level the board so the downlead is vertical, stick the ends in the holes, and read the differential pressure. take a couple readings.
hope it helps