New to tri

Hi and welcome!
I would post your question about which race to register for in the Triathlon Talk forum, too.
I think part of it depends on your personality. For me, if I registered for an Olympic and then changed to a sprint later, I would have a sense of failure before I stepped up to the starting line. That's just how my personality works. If I register for something, that's the race I do. No matter what. But it makes more sense rationally to do what you are thinking - train for the longer race.

On the other hand, it might make sense to do a sprint first just to get a feel for the rhythm of the event and the transitions and equipment, and then do a longer one a few weeks later, when you still have all the fitness from your training plan, but have more experience.