Initial phase for chirp function

I am working with logarithmic sweep in order to generate sound, and I really wonder if there is a mathematical reason to choose a cosine function over a sine function. Since in my mind, in order to avoid side effects, a starting value at 0 would make more sense.

Here is the function that generates the sweep (with default value for phi being 0) :

 y = cos(2*pi * (instPhi + phi/360));

When I think that this would make much more sense :

 y = sin(2*pi * (instPhi + phi/360));