I was using an LG V20 on Metro for quite a while. It's a very nice (and yes, big) phone. The cameras are good and give a variety of options if you enjoy photography. The small 'second screen' is gimmicky, but can be turned off. I could see that some people might appreciate it for notifications, but I didn't care for it. And as a flagship level device, there's a huge selection of cases available on Ebay and Amazon. Also, you don't need to seek out an unlocked phone; a regular T-Mobile phone will work on Metro.
While I'm not as familiar with the A20, reviews seem to suggest it has good battery life and decent camera performance, but may be laggy. It's also a large phone, but not nearly as wide as the V20, so it may be a better one-handed device.
I've alternated between current mid-range phones and prior flagships, and each has its advantages. If you like getting software updates and the newest little features, you may not be as happy with the older phone. On the other hand, if you appreciate a premium level device more, the V20 may be the better option.
LG flavors Android differently from Samsung's take on it, so if you're a big fan of Samsung, you might be happier sticking with them. One of my family members has made that choice because of the ease of transferring information from an old phone to a new one.
Oh - you will need to talk to customer service to activate a V20. The last time I did it by phone, I used my old phone's SIM and wasn't charged, but that may have changed. You'll definitely have to pay if you go into a store to activate it, so I don't recommend that route.
Hope this helps your decision!