Have the 2012 V6 and can't imagine a situation where you could or would want more power. Well maybe if i was drag racing. And the transmission seems to shift to the right gear when needed. Have no regrets.
I'm pretty sure the 2012 4.6 V8 is just so they can sell all the left over engines.
45hp and 33lb torque isn't going to be better than 100-200 lb less weight.
2009 Hyundai Genesis 4.6 Sedan 0-60 mph 5.5 Quarter Mile 13.9
2012 Hyundai Genesis 3.8 0-60 mph 5.4 Quarter Mile 13.8
Given there is no difference in performance now, the only choice is the 5.0 or 3.8.