The Rival 600 was my main for the good part of a year after it came out. I found it to be a very good shape. Only complaint was that the side grip eventually started peeling off during summer (this is very common on SS mice) however I was able to get full replacement modular sides for free from SS when that happened. I can't really go back to it now that I'm so used to lighter mice but I found it to be very good for the time I was using mice in the 90-100g range.
If you were using the old G502 before you'll probably enjoy the Rival 600, since you were using a heavier mouse before and aren't used to the ultralight stuff yet.
The Viper is a very different mouse, it's smaller, much lighter and ambidextrous. I have no doubt that it's an excellent mouse, the cable is supposed to be very good too, however Synapse 3 is a god awful piece of software...
I don't think you need to regret buying the Rival 600. Just play with it for a while and see how you like it. You can always buy the Viper later if you want to try it. Maybe there will even be some discounts on Black Friday.
Primary System Specs: Lian Li PC-O11 DX - ROG Maximus XI Formula - Watercooled Intel 9900k (4.9Ghz) - Watercooled RTX 2080 Ti FE - G.Skill Trident Z RGB 3200Mhz CL16 32GB - Corsair RM1000i PSU - Samsung 970 EVO 2TB NVME SSD
Secondary System Specs: Fractal Design Define S2 - ROG Maximus X Code - Watercooled Intel 8700k (5Ghz) - Watercooled Strix GTX 1080Ti OC - G.Skill Trident Z RGB 3200Mhz CL16 16GB - Seasonic X-1050W PSU - Intel 600P NVME SSD