If you get a warranty replacement from T-Mobiles team it should look like a new device. If not, there was something wrong and that should be addressed by them. Every replacement I got through TMobile looked just like new other than the box it came in.
That being said, some have gotten bad replacements but TMobile should make good on that.
You can go with Samsung but there you usually need to send in the watch and they fix it and send it back. If you can do without it for a while and want the original one you purchased, that is the way to go. You will probably need to be able to provide proof of purchase as well to Samsung.