Then most people are definitely wrong at level 10 and very arguably wrong at level 12. At level 10, Plant Form (Arboreal) gives you 2d10+11 as opposed to a TRex 2d12+6, gives you a better AC, and the same 15 foot reach. So, better to hit for more damage.
TRex is the flavour win. The level 5 Dinosaur Brontosaurus with the higher base
accuracy for 4d8+6 wins the damage while still having the reach option.