The tag should only be used for that first case. The second ("the question is about either A or B, but not both") is not how to properly use tags.
I reviewed site usage of that tag. The questions were all either about upgrading an existing bike, or about shopping for a new bike. I cleaned the tags up on all those questions. I also made "new-bike" a synonym for "shopping", figuring that's what a lot of people were trying to find when they applied that tag.
I also put this text in the tag summary:
Making a choice between upgrading a bike you have or simply buying a new bike
If you can phrase that better, feel free to make a suggestion.