Luke Whitelock is about as physical as a wet banana. I struggle to think of 6/8 in the country that is less effective with ball in hand. I have no idea what the AB's selectors (and Larksea) see in him. He has a good (but not outstanding) work-rate, generally makes his tackles, has a good passing game and can do a good offload now and then, and (apparently) has good leadership qualities. It wasn't as though he was even that good at Under 20 level - he was completely outshone by Rory Grice in 2010 and Brad Shields in 2011 (and I'm just considering at players who played 6/8 for New Zealand in those tournaments). It is not as though he have even been effective at ITM Cup level - Nasi Manu and Jordan Taufua have standout far more than him for Canterbury this season.
Luatua is far more physical. He may not be Jerome Kaino (yet), but he is far from Adam Thomson. I'm not sure whether you have been watching him this year Larksea, but a trademark of Luatua's game this season has been the metres he makes in close contact (in much the same way Kaino used to do). Luatua certainly needs to improve his physicality in defense, but he would still be a far more physical defender than Luke Whitelock.
Just a few stats from the 2013 Super Rugby season (courtesy of Foxsports.com.au):
L Whitelock: 717 mins played, 82 runs, 7 tackle busts, 4 offloads, 2 linebreaks, 0 linebraks created.
S Luatua: 1055 mins played, 115 runs, 19 tackle busts, 15 offloads, 5 linebreaks, 5 linebreaks created.
I think that emphasizes pretty well how effective (or otherwise....) Luke Whitelock is with ball in hand compared to Luatua....