When plants are flowering, they need less amounts of nitrogen to produce proper buds. The first number (the first 10) in 10-15-10 stands for nitrogen. The second number stands for Phosphorus. What 10-15-10 means is that there's almost as much Nitrogen as Phosphorus (10 parts nitrogen to 15 parts Phosphorus).
When flowering your cannabis plants, you're are looking for nutrient ratios where the second number is much higher than the first number. Of the two that you mentioned (Shultz and Miracle Gro Bloom Booster), you will definitely want to use the Bloom booster because I believe it is 15-30-15. I have never personally used Bloom Booster though. As far as nutrients, I have had great luck using Fox Farms nutrients, though I plan on trying the General Hydroponics three part nutrient series next on the recommendation of a fellow grower.
What are you growing in, soil or hydro?