Personally I think that in this scenario the PO is the better watch. Besides, the time for buying a good example of a 1st gen PO is now. I see prices creeping up and good full sets of a 2500D PO are getting close to the 8500 variants. If you really can't have both, I'd say do it. The BB58 can be reacquired in the future as it is not a discontinued watch.
I have a 1st year Blackbay. I like it, but doesn't wear it much anymore. The BB58 has some significant design improvements, but what makes it (in your words) near-perfect is that is getting close to the watch all the fans would want Tudor to make (again): A proper Submariner. In that sense I feel the PO has a more defined personality.
I have a 2201.50 so, regarding what to expect... The 2500 PO is the thinnest and lightest, but it is still a tall and heavy watch. In size and thickness I find it wears a lot like a 5-digit Sea Dweller, which kinda makes sense as it is the deep-diver Seamaster. In weight, the PO bracelet is significantly heavier (excellent, though).
You say you had a 2254 SMP before, expect the PO to feel heavier and significantly taller on the wrist. Same for the Speedmaster, the PO may look somewhat similar but wears nothing like it.
That said, I love the PO. It is a serious diver's watch with a lot of heritage and personality. And the 2500 movement (try to find a 2500D) is considered to be excellent and time proven.
I'm wearing mine most of the time since I got it in January. It's been one of the longest honeymoons I've had with a watch.
Regarding the comment by Alfista88 about the AR coating on the outside: it makes the crystal disappear ! I love the look. I have a couple of scratches on mine, but the fact is that you can always polish it off if it gets too obviously scratched.
Some pics to illustrate what I said (my wrist is 7.25 to 7.5" depending on the season)