Don't think there will be any difference in security between the 9a and 10 Pro.
If there's any propriety software you need for work, you might want to make sure that works on GOS first.
I would highly recommend switching to GOS overall. I actually switched GOS on my main phone but kept around my old iPhone for some work stuff, although I really don't need it all that much as I'm usually not expected to be available if I'm not at my desk.