Start with the mounting system. Tailfin offers frame and rear-axle mounts; the axle option requires an axle matched to the particular bicycle. Existing panniers from other brands need separate adaptors for this system. Compare the complete setup, rather than the price of the rack alone.
Next, check space for the tyre and brakes. Tubus technical drawings specify installation height, width and maximum tyre width. Frame material also affects the mounting choice: Tubus clamp sets must not be used on carbon frames.
Pannier hooks must match the tubing diameter. ORTLIEB Quick-Rack uses 10 mm tubes, while the XL version uses 12 mm. Their load limits also differ: 20 and 26 kg respectively. More capacity does not automatically permit passengers: child seats and dog baskets are not allowed on the Quick-Rack range.
Several Tubus axle adaptors and foot extensions restrict the load to 25 kg, even on a rack rated for more. If the rack itself has a lower limit, that lower figure still applies.