Which model of iphone 7 plus is better A1661 or A1784?

I would get the A1661 as it will work with any carrier. So if you decide to change carriers to get a better deal you can take you phone with you.

I got the A1660 iPhone 7 in 2016 and probably will be changing carriers as the coverage in our home is terrible and I don't have WiFi calling (the carrier doesn't offer it on the less expensive plans).

Purchase whichever model is for your region/county.

iPhone - View countries with supported LTE networks - Apple

They are both the same iPhone, so one can't be better than the other.

Depending on what you are going to use it for, the 1661 can work with all carriers, GSM and CDMA. The 1784 only works with GSM carriers.