In high-extreme volatility slot especially for Money Train family, I prefer to use buy bonus than normal spins. Reason is simple, because of my own experience. Getting FS in these games is rare or harder compared to medium-high volatility slot, most of the time we need to play hundreds of spins (we can lost more than the buy bonus cost) for a single FS but once we get the FS, the result is so bad. Obviously it is about preferences, but I still think that buy bonus is better than playing with normal spins for high-extreme volatility slots.