The main difference between the two postures is the position of the legs. In Konasana, the legs are parallel to each other, while in Utthita Trikonasana, one leg is extended forward. This can affect the stretch and engagement of different muscles in the body. Konasana tends to emphasize stretching of the inner thighs and groin, while Utthita Trikonasana places more emphasis on the hamstrings and hips.
Both postures can be beneficial for improving flexibility and mobility in the hips, legs, and torso. They can also help to improve balance and alignment, and relieve tension in the muscles of the back and shoulders.