depends on how quickly you get there after graduation
The best advice, bar none, that can be given on this topic. Take the first class available and report a couple of weeks early and you will graduate a year ahead of your classmates who lolly gag along.
Everything else you say is also absolutely correct.
First off, I am not sure Pax has yet flown a P-8. It will not get ahead of schedule. There will be budget issues, performance issues, contract issues, etc etc A plane this complex and an order this large will take several years to deliver. First off, when you graduate from flight school, if you are lucky, given possible delays, you will be given a choice between the P-3 and the P-8 FRS. Grades are paramount. If you select P-8, you will eventually report to one of the first P-8 squadrons. If you select P-3, you will train in the P-3. On completion of FRS, you will be assigned a squadron. Some will transition sooner than others. Make your druthers known. Grades are not necessarily paramount. In any event, you will report to your P-3 squadron, train, fly, deploy, and attain your aircraft commander and mission commander qualifications. At a determined time, the squadron will fly their P-3s to Davis-Mautham and report en masse to the FRS to be retrained in the P-8. Upon completion of the entire squadron finishing retraining, they will be given their new aircraft.
Flight time is flight time. In 30 yrs, people will view your P-3 squadron patches with awe. Old planes require a lot of maintenance. The mechs know how to work on old planes. New planes are never on schedule. New planes have a lot of bugs. Tech reps are the only people who can make them fly.
If you are lucky, you may have a choice. If you are smart, you may stay with P-3s as long as you can. There is always the next tour where you can fly it once the bugs have been worked out.