Iphone 6 plus will not default to SMS when other user iphone is not on wifi

If the sending phone has no 3G or wifi coverage it will send as an SMS message. Otherwise it gets sent as an SMS. If the receiving phone doesn't acknowledge the message after 30 seconds (because it doesn't have 3G or wifi coverage, or it's turned off) the sending phone will give up on iMessage and send it as an SMS.

If the sending phone has turned off the setting to "Send as SMS" then it won't send as SMS unless the user manually does it.

EDIT: If the receiving user has Messages set up on their Mac running Mountain Lion or Mavericks, and the phone is out of data coverage, the message could be received by the Mac and not the phone.

As far as I know you can only do that manually.

There are few differences between a Text Message and an iMessage.

1. The most obvious an iMessage has a blue colored background and a regular Text Message has it's background green

2. The name of the contact/number you are sending to will turn blue or green respectively for iMessage and for a regular Text Message

3. After resolving the phone number of the receiver(described above), the text box will automatically say what kind of message will be send and the "Send" button will be colored blue or green respectively for iMessage and for a regular Text Message