I have three A90 amplifiers and all are within .1db until 15 percent volume and then perfectly aligned for channel matching. I just personally like them better than the A90D because of the dot matrix display and remote aren't needed for an amplifier. I've had so many of both brands over the years, the fail rate is the same (usually 1 percent with computers and electronics). Topping updates more frequently, with SMSL you're pretty much assured they'll come out with a new version before doing a firmware update, they rarely release them. Topping designs are more elegant, SMSL is more utilitarian, but it depends on the model. I have the SMSL SP400/M400 stack and also an A90/A70Pro stack and the fit and finish on all their top tier products are excellent, but SMSL uses more plastic and the gauge of sheet metal for cases is thinner (push on the DL200 and it'll flex).
Topping has nicer design and finishes, but SMSL has gotten a lot better with the DO400 and DO300EX which I've got also. Both have a very clean looking and nice IPS display. Heat is a big difference between them, each has different models that can heat up a room by 1 degree F if left on all the time. Not as bad as Gustard like the X26 running two 9038Pro's in current mode instead of voltage mode, and will bubble the finish on the right side if left on over time in a cabinet and not in front of a fan.