I don’t think the sim free model for XR would be available in the U.S during the launch and likely won’t be available at the Apple store after 1 month of its initial launch. There is no option on the XR to choose the sim free model. However Best Buy does sell the,pm but you need to buy it full and be activated with Verizon SIM card to remain unlocked. I believe they will just have one model umber for the XR so doesn’t really matter which one you buy.
If you buy from the Apple store full price, it will be unlocked and doesn’t matter which carrier to go with.