All four major carriers are offering some kind of promotion, or competitive pricing, in addition to LG offering a headphone promotion with the V20.
Whether the promotion or competitive pricing is best for you depends on what kind of phone you would have for trade-in, and if the carrier will accept the trade-in.
In my case, I have an old S4 that I will use as a trade-in with AT&T, and get 200 dollars off the 830 V20 price they are offering, which would effectively make it a 630 phone for me.
Verizon offers the V20 for 672, but wouldn't give me a trade-in value for the S4. Odds are good that the phones Verizon will accept for trade-in, you could probably get a better price on Ebay. There in the case of Verizon, the reason to go with them is because the 672 is the most competitive price without a trade-in.
T-Mobile and Sprint have similar or variations of those deals.
T-Mobile has already started shipping the V20s, AT&T will start shipping in a few days.
If someone is coming from a Note 7, I would not use that as a trade-in, but simply get a refund, and then decide what to do with the proceeds
Just my two cents