I've not tried the Rivet Pearl. I've professed my love for the C17 carved here and on the iBob list a few times. My previous saddle of choice was the B17n (narrow) whereas the B17 was too wide, so, I'd say compare the widths first with any other saddles you currently ride. I believe the C17 is a bit narrower than the Pearl. If you're sensitive to width, the decision may be made for you.
After finding nirvana with the Cambium C17 carved, I have gone back and carved one of my B17n's, and it's a better saddle now as well - the cutout improves an already great saddled.
So... cutout = good. If width for either = good for you, it comes down to materials and/or construction maybe.
I've found in my personal experience that the C17 carved flexes just enough, just like a well broken in Brooks saddle - not an over-used too-broken-in saddle. What the C17 doesn't do is flex or sag too much when it gets wet from precipitation or from a long day or series of days of being sweat upon. Sometimes, my B17n feels a bit saggy at the end of long, hot days. When combined with my own fatigue, it can diminish my ride enjoyment. I'd suspect that any leather saddle could behave in such a way - at some point, too my wet = bad with leather saddles.
I like that I don't have to think about precipitation at all with my C17. I don't wear myself out thinking about such things with my leather saddles, but I consider it and carry a saddle cover or shower cap for them when there's chance of rain.
I've heard feedback, on this list and iBob, that the C17 is prone to cracking/splitting towards the nose of the saddle, near the frame support there. I haven't experienced this at all, but it's possible that these have a material durability problem.
I have noticed what appears to be a decline in quality with Brooks leather saddles over the past few years. If I were to look to purchase a leather saddle, I'd look to the Rivet saddles, based on the positive feedback I've seen here and on the iBob list.
-Jeff
Silver Spring, MD