Will I be able to leave my 6+ in the car while I'm shopping, and just scan my wrist at checkout? This would make me far less concerned about carrying around an iPhone that is so big. Might even be the deciding factor for me.
The Watch has an NFC chip to facilitate Apple Pay transactions. However, the authentication is being accomplished by the iPhone over the Bluetooth link. As long as the Watch is linked to the iPhone, you'll hold your wrist close to the NFC scanner to complete the sale, leaving the iPhone in your pocket or bag.
The key is what the OP means by "nearby". That’s a very relative word. My guess is that you must have the phone within about 10 meters (33 feet) or so - which is the standard bluetooth range.