I think window first is best, because it is more likely to become unopenable as the vehicle sinks.

They are saying it takes between 30 seconds and 4 minutes for the vehicle to sink (which sounds about right, normally - I think the recent bridge collapse was a worst case). So you should have a few 10s of seconds before the water reaches the level of the window. Once that happens it becomes harder to open them, because the pressure of the water holds them closed. Also, it's harder to smash a window when there's water on the other side of it. It's also harder to smash when the cabin is full of water because you can't swing your arm properly.

Getting the window open or smashes is urgent. After that you can spend time fiddling with the seat belt. The cabin will take a while to flood even with the windows open.