I like walking, too.

If one is able to get a seat in one of the front BART train cars, G check-in is obviously closest.

But if further back, and laden with heavy luggage, it may be easier to take the AirTrain to T-3.

A big factor is the relative length of the check-in and TSA lines. But is there any way to compare those in advance?

If not, go with the better odds.