We are a family of six and we prefer to book a suite to ensure that we are all together. Being together (connecting rooms) is really important to us when we are on holiday so it's something we prioritize.
We haven't stayed in the Princess Suite but we have stayed in the King K suite a few times which is located next door to both Princess Suites. I think the floor you are on is determined by whether you are staying in the two bedroom or the one bedroom Princess suite. We thought the view was spectacular from both floors - it was a full on centered view of the castle and fireworks. Really pretty! But, I know that Hawaii has great views too.
Whether you check in at Tonga or not may depend on the time that you arrive. A concierge is not always on duty in Tonga. We have checked in at Hawaii and in our room at Tonga - we arrived a little late one night and a concierge met us at the front door and took us straight to our room, had us sign a few things, and just handed us our information packet. It was very nice.
The food variety is far superior at Hawaii, IMHO, and it isn't inconvenient to walk between buildings but we didn't use the lounge in Hawaii very often - I think maybe two or three times. Breakfast is available in Tonga and there are snacks and drinks in the lounge all day and late into the evening. The lounge is very small and we would just pick up our things and take them to our room - we thought it was great!
Also, because the building has so few rooms the concierge service is very personalized and that's something we really appreciated. It feels very intimate and almost like you have your own private home at Disney. But, in the interest of full disclosure, we did find that even normal level conversational and tv noise traveled easily between the rooms (nothing that bothered us but we did notice it). This may be the case throughout the Poly(?) or is specific to Tonga - I don't know.
We also loved the location of the building - it is connected by covered walkway to the GCH which is terrific when it's raining! It is also super close to the pool (and the bar

).
I don't know if this helped very much but we loved it, and a suite in Tonga is our first choice when we stay at the Poly. But, having said that, Hawaii is wonderful too. IMHO I think it's really personal preference - you simply can't go wrong - you are going to love it in either building

!