It's been my anecdotal experience (roughly 75,000 spins) that Spin Poker has lower variance than 10-play. And I believe the reason is how the draw cards are selected. Each drawn card can appear multiple times in its own column, but cannot appear in other columns once it appears. So when holding a pair and drawing three, the three selections in the final column are selected from as few as 41 choices (rather than the 45 each individual line would have in 10-play).
Yes, when drawing things like RF4, the covariance is large since any hit represents 2,3 or 5 Royals depending on the line placement. But there are a lot more times that you draw to a pair or three of a kind that SP's draw method has a smoothing effect on the results.
I may be a blind man describing an elephant. But that's what I have seen.
My thinking is that 20 royals would be about right for 10 play, but I think spinpoker has more variance than 10 play. Maybe 30 royals for spinpoker.