From my personal experience dealing with these companies, both are constantly striving to improve any glitches that their new products may have. JDS is willing to customize their firmware and devices to a higher degree (personalized engraving, adding more weights inside plastic cases etc.).
The main difference between the two is the fact that JDS takes longer to bring new products to the market and tends to move more gradually to newer DAC chips and op-amps. Their products are a little short on extra features like screens, user-selectable filters, balanced outputs, DSD, MQA, but big on reliable, glitch-free performance. For example, they include a linear power transformer rather than rely on USB power with their DACs. Their warranty is also twice as long.
Topping is very ambitious with their constantly expanding product line and, thanks to their products being available on Amazon, risk-free in terms of dealing with potential returns. Their products are more for enthusiasts who like to experiment with different DAC chips and filters as well for people who really need DSD, MQA, balanced connections and all those exotic inputs. In terms of reliability, I think they are very good as well. The only well-documented problematic product they had was the first version of DX3 Pro, and it was fixed with the second revision. Their products have a slightly higher incidence of little software glitches (my D70 had to be rebooted once every couple of weeks because it would freeze when switching sample rates), but none of them have been deal-breakers in my opinion. There are also some counter-intuitive engineering choices like having an OLED screen on a device that displays a static screen 99% of the time.
In terms of dealing with customer service, JDS is the best in the industry.
@jseaber responds within hours, and for people who are located in North America, their RMA process is super smooth. I have never really had to send anything back to Topping directly, so I cannot say how that process works.
@JohnYang1997 has been very responsive when it comes to addressing any product questions here though. I would not hesitate buying from them solely based on the fact that they are located in China.