The two methods do not produce exactly the same results, however with respect to the amplitude of the values in the plot, they only appear to be the same.
Do this experiment to see how they differ:
x = 0:pi/4:2*pi;
v = sin(x);
xq = 0:pi/16:2*pi;
vq1 = interp1(x,v,xq,'spline');
vq2 = interp1(x,v,xq,'cubic');
plot(x,v,'o',xq,vq2,':.');
difvct = vq1-vq2;
figure
plot(xq, difvct)
grid
.
