Just finished rebuilding one of each... both among the best compressor pumps available today. Both are easy to get parts for, with similar costs.
However, there are some key differences:
S-B pumps are all cast iron, with removable cylinder heads. Champion uses a one-piece aluminum cylinder assembly with steel liners.
They both use easily serviced disc valves.
S-B utilizes cast iron intercoolers, Champion uses the more common finned copper tubes.
The rotating assemblies are both very similar.
In general, a S-B pump will run cooler and at a lower RPM to achieve the same volume as a Champion.
If I had to choose one, it would be a Saylor-Beall. Slower, quieter, heaver, more thermal mass... will probably outlast me.