The main difference between SUP and ISUP is that SUP boards are wider and more stable, making them better for beginners. ISUPs are narrower and more agile, making them better for experienced paddleboarders.
Both types of board have pros and cons, so it's ultimately up to the individual rider to decide which kind of board is best for them. It comes down to personal preference. Some people prefer the stability of a SUP, while others prefer the agility and speed of an ISUP.