Here's the thing... 10mm Auto is awesome!
Is it more expensive? - YES~ A LOT MORE
I personally would not even consider a 10mm unless I reloaded for it.
The other thing to consider is that store bought -cheap ($30/box)- generic 10mm ammo is no different than .40S&W in terms of POWAH!
Now, reloaded ammo (or defensive 10mm Auto ammo - which I have yet to see in a Canadian retail store) is awesome in terms of power and dwarfs the .40S&W.
So... my advice is to start reloading!
It's actually very easy for pistol cartridges and fun... the money you end up saving is incredible especially when you are talking about the more expensive rounds (10mm, 44magnum, etc.)
Recap:
Glock20 is the best Glock... 10mm Auto *can* be super powerful. I wouldn't get one unless I reloaded for it.
~Cheers
Note: If money is no object then by all means, by your retail 10mm by the thousands! Get the hot retail stuff too

Wish I could.