Have you browsed through the Verizon Wireless Community forum and seen what other discussions are about those particular models?
I'm most definitely NOT a VZW employee. If a post answered your question, please mark it as the answer.
The J3 V 3rd gen is new( like within the last week or so ) but seems to have slightly better specs than the J3 V2nd gen. Unless you just don't like a larger screen I'd pay the extra $3 a month for the J7 V 2nd gen or the Motorola G6. And between those 2 probably the G6. But of the 2 you listed I'd probably go with the J3 V. The K20 is an older phone.
for $7 a month you're getting bottom of the line phone!!
depends how you intend to use the phone & capabilities are offered on that particular phone!!