OK, here the deal, impact drivers do it better. They do just about everything better even driving long wood screws into decking. So, with that being said, what do you do more? In mechanical work you will do more nut driving then anything so an impact driver is the way to go. You will find that the impact driver can drill too, but with a drill with hex shaft or a attachable chuck. WARNING, an impact driver can drill, but the torque is different and what that dose with a drill bit, is it has a tendancy to drive the drill through the metal before it has time to actually drill the hole. This has the effect of driving the drill through the metal before you have time to pull it out and can be disasterous with a refrigeration system or electrical components on the other side of the metal.
So, what to do? I have the answer to this dellema. Panosonic makes a combo tool that fits the bill, plus its batterys will out last any other cordless tool on the market. Here ya go...
I have this tool and it kicks butt, it's even a better drill than most drill I have owned (drill mode) and its a hefty impactor too at 15.5 volts. It's heavier than most other impactors but it's got guts and guts is enough.