Do you have a set of .45 ACP dies now, or will this be your first set?
Redding makes some very nice dies. Dillon's are great, too, though, and they give you what you need to reload on a progressive machine.
For a 550B, you want a sizer/deprimer, a seating die, and a crimping die. (You don't need an expanding die -- the powder measure takes care of that.) If you buy the Redding dies, you'll wind up with an expanding die you don't want and you'll still need a crimping die.
After about five years of reloading .45 wad ammo for Bullseye competition, I've found that ammo loaded with my Dillon dies shoots as well as anything else. The most important factors in making good Bullseye ammo are:
1) Bullet selection
2) Consistency and quality control
3) Finding the combination of powder charge, overall length, and crimp that will produce the best reliability in your pistol
I say buy the Dillon dies and start loading practice ammo. When you're shooting 98-4X slowfires and looking to get the competitive edge over the other high master shooters, then it's time to worry about your dies.
