Originally Posted by
juanebici
not sure what the difference between the Senior and the Master category is
Just to directly answer the question, in this context "senior" just means "not junior", in other words anybody over 18. "Master" means "old", in this case 40-49 years old.
If you can only do one, the Masters race may be a better experience (no guarantees though). If you can do both, even better, the races are short.