60A max, MCA of 38 means that your 40 amp service is "ok".
I prefere to go with the maximum rated size whenever I can though.
HVAC equipment is hard on breakers due to the high inrush current of the compressor. The 40A breaker is designed to handle that, but the breaker is much more likely to fail a few years down the road than a 60A breaker would be.