If you are sure you will get the RG MOP butterfly ring to wear it with your Serpenti ring, I think the Serpenti earrings will be perfect. The earrings will make a perfect matched look with the Serpenti ring, and the butterfly ring will be special accent piece.
The RG MOP butterfly earrings will work, but they are not as nice as the diamond butterfly ones in my opinion. To me, the MOP stone changes the whimsical butterfly look to more of a cutesy butterfly playset look, a bit childish, as there are so many MOP butterflies now, and MOP can look like plastic-y in a cutesy setting such as butterflies. I hope that made sense. I am sure once you try on the MOP earrings and ring together, you will immediately know if you love it or not. Jewelry is all personal preference.
Speaking of earrings...
I recently lost one of my WG Frivole pave earrings. I just bought a new pair, but that was the easy part. I now have to annoy myself and go through the "adjust my earring" drama again.


Not looking forward to it.
Back to your earrings...
Since you have the Serpenti ring, I would go for the matching earrings. I assume you have the full pave ring, and the "scales" kind of make the ring look like it's bezel set diamonds, even though it's not. The bezel edge though will look great against the similar bezel edge of the butterfly, especially the MOP.
But... full circle back to your ring...
The RG MOP butterfly ring is nice, and I am sure you know this, but with a MOP ring (or any pearl ring) you can't exactly wear the ring when you're roughing it out. I find pearl bracelets and necklaces easy to wear, and I wear them even when I'm roughing it out. However, hands get banged a lot, so pearl/MOP rings aren't quite everyday rings vs. the sapphire ring as sapphires are pretty durable.
If you are trying to find a RG ring to match your RG Serpenti, maybe try the RG pink sapphire butterfly ring. Of the two RG choices, for me, I would choose the RG pink sapphire.
Anyway, just my two cents in case for some thoughts in case it might help you decide. Good luck and congratulations on your soon to be new butterfly ring!