Round ligament pain is more shooting or stabbing. It lasts for a few seconds and goes away usually on its own or by changing positions. It's caused by stretching muscles as you grow and more common in the first and second trimester. SPD is long lasting, feels like overworked sore muscles, like over stretched or pulled muscles, stays in standing/sitting/laying positions and is more common towards the end of the pregnancy. It's due to the hormone relaxin over relaxing your ligaments.
I had lots of round ligament pain in the beginning. But honestly that was cake compared to SPD. The SPD started around week 29. I'm 33 weeks now, it hurts to roll over in bed, squat, sit down, stand up, walk, especially upstairs, pretty much always there.