Plot both solutions with points instead of solid lines. What do you see? ode15s solver is appropriate for stiff ODEs. This means that the solver can adjust its step-size to be small when accuracy is needed in regions of rapid changes and to be bigger in regions where the solution is changing slowly without worrying about having to maintain stability. Even though the cost per time step is generally more expensive than a non-stiff solver, the stiff solvers usually are able to take bigger time-steps on average, and as a result solve the problem quicker.