I am wondering why I noticed in a couple of the Island speed runs people chose rexes why not allos the reason I think allos ( again I am a noob here and am probably missing a few things) is because of turning radius is allot smaller and they can get through caves allot easier and probably faster

People prefer Rex for several reasons.

1. Rex, when attacking, pushes the bitten target back, not allowing it to close the distance
2. Rex is easier to tame
3. Rex is stronger and has more health
4. Rex is a larger predator - few people will fight with him. For example, Purlovia will never attack a Rex rider.
5. High maneuverability - i mean he can byte nearly 360 degrees
6. It is possible to walk on his back. It also allows you to move without spending stamina (Command whistle).
7. Rex easily eats flying small livestock attacking the rider
8. Rex is beautiful

Also rex can walk into Tek Cave ... Spino can not do it. But it is possible to get spino in tek cave.

IO discovered that Spinos cannot get into tek cave I see what you mean about Rexes double damage when compared to allos I put the same amount of points into damage and yes rex has allot more my last successful run I couldnt find any rexes which is why I tamed a spino and of course after I tamed a spino ( only to discover later they cannot get through Tek cave door) I found rexes every where