Well, the .40 S&W round operates at a higher pressure than .45acp. Most folks find the recoil of the .40 to be snappier than the .45.
I use to have a S&W 66 myself, nice gun! Keep in mind if you ever have to use a gun, short of an all out zombie attack, the chances are pretty good you won't need anywhere near 50 rounds. Don't worry too much about how much volume you can shoot at the range, when I take my .38 snubby to the range I rarely shoot more than 50 rounds and sometimes not even that. For the 66 I'd recommend using .38spl for the majority of your practice anyway and just enough magnum rounds to keep your proficiency up. Most of us who carry .38spl, .357mag, 9mm, etc. usually focus our practice on std. pressure loads and carry +P for defensive use. Actually, the only caliber I DON'T carry +P defensive loads in is the .45. Hey defensive ammo is too expensive to practice with all the time anyway.