Air duct leaks..more loss in winter or summer?

It depends on the climate and which get used more 

since you didn't tell us what your climate is, we can't say other than "both" should see savings

As a general rule air cooling is more expensive than air heating, but it actually depends on specifics for your situation.

It really depends on where you live. In more northern areas the winter heating load tends to be much more than the summer cooling load. In more southern areas the cooling load in summer can be much higher. 

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Where I live in East Texas Nov, Dec, Jan are the highest loads  even more than Jul or Aug in the summer. 

the savings overall may be minimal, but helping your HVAC to work more efficiently is always good.  Be sure you insulate properly so that moisture will not build up between the ducts and the insulation causing mold.  

Without SOME hint at local weather conditions, your target internal temp, and the efficiency of your HVAC system, flip a coin and you will get as good an answer.

Probably more savings in the winter because warm air is cheeper to make than cold air .