Both companies have some things that the other either doesn't or that 1 has that is better.
RCBS for example makes dies and presses for 50 caliber , Redding does not.
Redding makes about the best match dies available, RCBS "match dies" are more generic than match.
Redding has no prblem making up a weird die set, at slight additional cost, RCBS is harder to get true custom stuff from.
Both have truly excellent customer service.
To have some products from both is in my opinion a good thing.