There's enough of a difference for it to be noticeable. At -112 dbm is when calls can begin to drop. If the iPhone XS sits at -110 dbm on average in my house then there are times when I couldn't make or receive calls.
In a lot of areas Verizon really isn't that much better than T-Mobile or AT&T. I left Verizon because of weak signal in my area. Once they turned on their new XLTE, as they like to call it, bands signal strength, range, building penetration went down. LTE band 4, Verizon's new one, doesn't have the range or building penetration of band 13 so region where they turned that on and program the phones to connect to band 4 over band 13 will have decreased range. That's what happened in my area. I had dropped calls all through my town. Comparing signal strength will actual numbers T-Mobile is on par with Verizon, both networks average -95 dbm in my area. Verizon may cover more fringe areas but it's definitely not worth wasting $200 more a month to go on Verizon for slightly better fringe coverage. The Verizon coverage got so bad and I was able to prove it that I was able to be let out of my contract with penalty. Networks and manufactures can manipulate the number of bars a phone will show but they cannot alter the dbm. That's how I know the iPhone XS antennas are weak. For two of them to perform the same with the same results is not a coincidence. Add in that all the other phones in my house get a good signal in my basement and the only one will no reception is the iPhone XS and that's pretty damning evidence.
Your area my be okay on Verizon, mine on T-Mobile is not, but the initial reports are a good percentage of early adopters having issues. Another add in is the Wiwavelength blog on September 12th that basically predicted this. For me, adding all this up and looking at it logically is enough proof that sending back the iPhone XS is the right call.