The 12 volt should be a little lighter, but my new 18 volt ion dewalt weights a whole lot less than my older 12 volt dewalt. Just depends on what you're using it for. If the screws are more delicate, the 12 volt may be a better option, but both should have a torque setting to keep you from twisting any screws off.
Also you may look to see what other tools use the same batteries, down the road you could pickup a saw or some other tool at a cheaper price without the batteries/charger and use the same ones from the drills.
To a small child, the perfect granddad is unafraid of big dogs and fierce storms but absolutely terrified of the word “boo.” – Robert Brault