[Zeke]

If you have to increase the main service to accommodate more breakers, I would first consider the sub panel. However, the main has to still have enough capacity for the loads. You can't keep adding 240v circuits forever.

The way I understand this is that you are just replacing the condenser unit, so the load isn't that much more. If it boils down to spaces for breakers only, by all means put a sub in. The nice thing about a sub in the same building is that you don't need new ground rods. Just unhook (or don't complete) the bond between the ground bus and the neutral bus.

Calculate the total load on the sub when choosing the breaker to feed it. Most say that the running amps is just enough, so you don't need 70 amps to feed a 40 and a 30 breaker. Hopefully your new condenser has a soft start anyway. But, given what size breakers are available for your main might be the deciding factor. In that case choose the larger. You may want to add another circuit some day. Like a car charger.