Given the choice between the A3 and the A5, I have opted for the A3 with an adjustable cheekpiece. Here's why....
When shooting prone and making elevation adjustments the bottom of the A3 buttstock is 'straight' so a rearbag slides smoothly forward and back. Whereas the A5 has a cutout that does not allow for as much adjustment.
Both of my stocks have the adjustable cheekpiece. Maybe it's just my physiology but I prefer to have the cheekpiece pretty high and an adjustable cheekpiece is the only way to get that high (my rifles have Leupold MK4s with 50mm objectives, so they are 2" above center of bore).
In terms of positional shooting, both of my longrange rigs are regularly used in tactical competitions. This means shooting from standing, kneeling, sitting, prone, urban prone, off barricades, off trees, off a pack...all kinds of crazy positions. The A3 performs very well in all positions.
As I said, given the choice, I take the A3 over the A5. I talk the talk and I walk the walk....
My 7saum by Hawk Hill Custom
My 260 by MCR