Go with the nephew, since most songs you will be playing that use a "G" chord will also use a "C" chord and the transition between those two chords is MUCH easier when you are playing the pinky version of G.

I too was first taught the index-finger version, but was "corrected" by my cousin, who showed me the pinky way. I immediately learned his version (which is odd, in retrospect, since we so seldom agreed on anything) and haven't looked back.

Which in a way is bad, because now it takes me a long time to get my fingers into the other chord, and as some people have already said here, there are times when the index-finger version would make for smoother transitions.

So retain the ability to do it the other way. But I would guess that about 90% of the time, at least on beginner and intermediate level songs, you will be using your nephew's version more.

Alex