As a service guy who comes along after the install.... USE THE LARGER WIRE SIZE!!!!!.... The smaller wire size generates more HEAT. The large amp draw causes the small wire to act like a heater element, and, overheating problems occur at the termination points.
Copper is an excellent conductor of heat , oversize wire sizes will draw heat away from the termination points.
The smaller wire has a higher resistance and causes the compressor to work harder. Think of running two light bulbs in series, a element will burn out quickly because of the additional amp draw of the other bulb.
The old industrial rule of thumb is run the wire at 125% and fuse at 80%. The fuse should blow before the equipment tears itself up.
If a problem occurs such as low freon or control failure , then the fuse should blow before the compressor overheats and shorts out.
A recent troubleshooting call proved to me that 14ga wire can only pull aprrox 28 amps, it was a dead short, "Down the line" and the amp gauge would hold at 28-30 amps,then the breaker would trip in approx 2 mins.
Is customer is paying you to "do a good job that will last a long time" and provide enough room to upsize if necessary.
---OR------
Is the customer paying you to " do the minimum amount and let the next guy worry about it" ??