What's going to make me more power? Colder air sourced outside the engine with a longer intake tube, or warmer air sourced inside the engine bay with a shorter intake with less restriction?

 

The coldest air you can have at the inlet of the turbo will yield the highest particular mass flow of any combination.

 

Cold air
Large diameter tube
Minimal bends to create turbulance

 

Cold air will always be the best for any cars made. your car will run better when it cooler outside than the summer time.

 

I will agree with what has been said so far, but it all depends. What temp differences and what pressure differences? If you know those numbers, you can calculate it yourself.

I remember doing some calculations a while back and came up with about 25* increase in IAT was about equal to a 1psi pressure drop. Here's a post from a while back where I did a back to back test of the stock airbox and open filter. This is a good comparison of pressure drop vs temp comparison.

But, like I said it all matters how much pressure drop you are adding with the longer pipe compared to how much cooler temps you are getting. If my math was right when I was calculating it go with the 25*F per 1psi as an equal tradeoff.

 

On some turbos (think Black or equivalent) 40-50* reduction in inlet temp can be 7 lbs/min. or more.