Mercy and Grace are 2 sides of the same coin. Mercy is when God does not give us what we deserve, i.e. punishment for our sins against Him and others. Instead He forgives us for our sins. Grace is when He gives us what we don’t deserve, i.e. the assurance of everlasting life in heaven with Him and the strength and wisdom and the heart to pray to Him, to wait upon Him and cooperate with Him so that His plan for our life bears fruit. So actually, it’s not Mercy vs Grace, but Mercy and Grace.
The unique thing about the coin of Mercy and Grace is that it falls with both sides up, making you a winner 2 times over!