My ability as a triathlete is average and I would recommend taking none of my advice. However, my ability on a MTB is way above average as I have ridden bmx/MTB for years and raced motocross. 29er all the way! Opens doors for long distance races etc, will make life easier as a beginner (more confidence over logs etc), can be ridden hard and fast as they are so stable. The new models corner/ climb that good I can't see any negatives.
Buy a 4 inch dally with 120mm forks. XT group set/ brakes are amazing.
I do find at the car park people always chat about bikes etc and seem to know a