Spinos are better mounts than rexes, considering they are a great deal faster, more agile, and versatile on land and in water. However, due to their lower health, alpha boss fights will be considerably harder when you use Spinos instead of rexes, although Spinos have a higher DPS than rexes, with levels, rexes have significantly higher hp than Spinos, making them difficult to utilize against alpha bosses, however, from experience, the Spinos tend to do very well against gamma and beta bosses. They both have their pros and cons, but for me: Spinos for traveling and for meat runs, and rexes for boss fights.
To be strictly honest the spinosaur is one of the best mounts in game if you are even wble to obtain one be sure to make it a high level one they are very squishy at first so go around killing small dinos before attacking big prey
TLC patch coming in March 2018 to add the ability to swap between 2 legged mode (faster attacks and ability to attack with claws) and 4 legged mode (Faster movement), as well as increased turning radius, movement speed, damage, and health regen while in water and for a short time after leaving it. Like so people can see!
When in water, the spino can move without using stamina, this means that in a fight, if you run into a deep water source, you don't need to worry about stamina if you need to retreat. Has an oxygen bar so you can't dive forever sadly, very fast in water though.