Both true, but right now, when compared to Jaeger, Fortication doctrine doesn't look very attractive and even if one has it they would most likely still stick with Jaeger. I mean of all the Elephants you encounter, how many of those came from Fortification? I'm pretty sure it's less than 1%. Called 'fortification' or not, that doctrine needs an extra something over Jaeger, or Jaeger needs something less.
The main reason why you see Ele's from Jager is everybody has Jager Armor doctrine and no one is gonna pay or grind for a commander that does virtually the same