Depends on what you and your team “REALLY” know about the market/customer you intend to serve.
If the customer does not know "what the product provides", because he/she has no reference or experience, but you think you know, then the product comes first... this was Steve Job's view on what he built at Apple. He did not believe in User-groups, as he was creating something the users did not yet understand or comprehend till they had it in hand.
If you don’t “REALLY” know, then you need to tap the customer experience/insight. Note that in both cases he/she is often not after the product itself, but "what the product provides" him/her. So you could engage with the customer first to find out is the "what the product provides" that the customer is after, then build it.
eg. A Drill is a product that makes a hole... the customer is after the hole, not the drill. If you know he’s after the hole, build the drill, if you don’t know he is after a hole, ask him, then build the drill.