RumoredNow provides excellent and cogent points.
I have a 1520.3 on T-Mobile and live in the middle of Long Island, NY. All carrier signals barely reach my house (two bars) because I happen to live in a dead spot for all carriers (fine for every provider approximately after half a mile travel in any direction. LOL @me

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I have a work associate who was forced to move from T-Mobile to Verizon because he had no signal where he lived north of NYC closer to his job in CT. Metropolitan NYC no issue as the coverage is great and fast even in the boroughs. Long Island coverage is also good in my area.
The Note 4 is attractive to me. However, I am precluded from purchasing due to a significant financial investment in the iOS and Windows Phone ecosystems and the four month old purchase of my white 1520.3.
The real reason to get WiFi calling from T-Mobile for me is the excellent Asus RT-AC68U wireless router (200 USD on Amazon) provided for 25 USD.
The only additional hardware issue to me is the WiFi. The 1520.3 WiFi appears weaker to me as my 1020 attains a stronger signal. I believe this lack of WiFi signal strength is attributable to the different WiFi chip in the 1520.3 which supports the wireless AC standard. Unfortunately, it appears this hardware does not function as efficiently as my 1020 with my Verizon provided Actiontec MI424WI wireless modem/router (only supports wireless b/g/n standard).
Ultimately, due to the generally excellent T-Mobile coverage in NY, I believe this is a financial and ecosystem preference decision. I advise waiting until November when holidays deals will impact any purchase.