All the mini-suites are virtually identical; the only difference is location. Which deck is "best" is just a matter of personal preference; I like to be up high and all the way forward. Somebody else might like to be more centrally located midship or aft on a lower deck. Study the deckplans and see where you think you might spend a lot of your time.
On Princess people still tend to dress quite nicely on formal nights, especially on longer cruises (10 days or more). There will be a lot of long gowns and tuxedos but it will be mostly "dressy" dresses and suits and ties. "Smart casual" nights are much more relaxed: skirts and blouses and pants and nice tops for the ladies; nice shirts and slacks and maybe a blazer, with or without tie for the gentlemen.