Fire a few cylinders of .38 just to get your grip set, then fire a few of .357. I usually run about a 5 to 1 ratio when I fire mine. the .38 for fine-tuning the eye, the full-house .357 for effect and familiarization.
One of the joys of the .357 is that you can customize your loads. My standard load for the house is 2 rounds of .38 wadcutter, 2 rounds of +P .38 hollowpoint, and 2 rounds of 125 grain JHP .357.
Two for the hat, two for the cowboy and two for the horse? 
Pretty close. 2 for the Bad Guy, two for his buddy, two for the car. The wadcutters have very little muzzle flash, which makes them the better round for firing in a darkened environment. Then I up the stopping power as needed.
Lots of grip options for the SP101 Ruger.
I concur with silenus: Throw a couple handfuls of .38s through it to get the feel of the gun first. The magnums won’t be as surprising if your hands are familiar with the grips and the balance.