Well I have used all 3. First of all and most effective is Gravity. It seems to work a bit better in hydro then in soil but it does what it is supposed to...harden the buds! I have seen about a 15% increase in yield and over all quality of the flowers using gravity as its recommended.
Bush master is good if you have the right strain. If you have a fast growing sativa strain Bushmaster will definitely promote some lateral growth and stopping vertical growth. Which is handy in a restricted indoor environment. On the other hand if you have a strain that is allready stout and bushy in my opinion it has adverse effects.
Snow storm ultra is a potassium based enhancer used to promote essential oils and what not. I have only used this product as a foliage spray the last 2 weeks of veg and the first 3 weeks of flower. I have seen a slight increase in crystals. To sum it up coming from a person who has used all three I would say the only one really worth its price is Gravity.