Well, I have both the G20 and G29 and other than length and height, they are basically the same gun...well, they do use different barrels.
I've velocity tested both using Underwood ammo, and I remember the UW 155gr JHP from the G20 posting right at 800 lb-ft of energy, and the G29 posting 700 fpe. Now, say what one will about what the 155 gr JHP is ideally suited for, but whatever that is, from a G29 it will deliver the energy!
As for cost to shoot, yes 10mm ammo is more costly than non-magnum ammo, just as is the case with all other calibers, however the FBI in collaboration with S&W was nice enough to bring use the .40 S&W which feeds and functions absolutely perfectly from both G20 and G29 without even doing a barrel swap. Last time I bought .40 it was name-brand Remington 165 gr FMJ-TC rated to 484 lb-ft of KE...that's some pretty serious power for $16/box of 50.
Another thing about the G29 is that it can be "upsized" by simply installing a longer barrel and G20 magazine. So you have both a highly concealable 10mm that can also be used for open carry in the field if desired.
As for recoil, IMO recoil from the 10mm Glocks of both sizes is pretty mild even with Underwood ammo. I'd rate it no harder than that of a 9mm fired from an 18 ounce pistol, but probably less "snappy" feeling.