I've driven that route many times and I prefer to stay on the 15.
First of all, you will not have to slow down and dodge traffic as you exit the highway. Yes, there can be a little more traffic on the 15, especially when you get around the 10, but not enough to worry about.
To stay on the 215, you literally have to exit the highway and join the 91 highway, and that intersection can be a bear. Then you fight the traffic at the 215/10 intersection. Plus, after the intersection, the 215 goes through San Bernardino with only 2 to 3 lanes in each direction and it's always crowded and in need of repairs.
