A double stack mag carries lots more bullets, which is a GOOD THING.
A .45 double stack is real fat, which may be a problem if you don't have large hands.
A double stack .40 mag is easier to fit into a slim grip.
Either will work.
He needs a VPC approved "Bullet Hose", endorsed by experts. :)
I wonder if anyone ever designed a pistol/magazine which was single-stacked at the top but double-stacked further down? I would imagine there would be some engineering challenges, and it might not be possible to make the pistols/magazines ambidextrous, but otherwise I would think that might be a useful compromose between single and double stacks.