In my opinion, a twin flame is someone you share part of your soul with. You are like twins but from a spiritual perspective – as in aura’s, energy, etc. People who are twin flames are not always meant to be in a romantic relationship in this life.
Also, in my opinion, a soulmate is someone you have been in a relationship with in a past life. That is why they feel so familiar to you. And just because you were in a relationship with them in a past life, doesn’t mean you are meant to be with them in this life.
Neither is known for being stronger, they are just different things. You might feel a stronger energy with a soulmate based on that you have been in several past relationships with them. It varies.