Im sure you can find the variance of S21 online, but also consider that you are not going to get BJs as frequently and there will be bonuses for multicard 21s, as well as more frequent doubles which would increase your variance
Im sure you can find the variance of S21 online, but also consider that you are not going to get BJs as frequently and there will be bonuses for multicard 21s, as well as more frequent doubles which would increase your variance
Doubles are a little less frequent and of lower variance, because a lot of the high-variance soft doubles aren't used as well as most doubles against 9. Also a lot of multi-card doubles aren't used due to the the multi-card 21. The nice thing about the SP21 bonuses is that they are received without additional risk.
The DDR rule is awesome, as well as being able to resplit, hit, and double split aces. That adds a ton of value. Because of all this the standard deviation of SP21 is significantly less than that of a comparable game of BJ. But you give it back with the larger spread necessary to beat the game.