My opinion is that novotel is more 'standardized' while mercure has more individual hotel's own character.
And since there are so many novotel and mercure, obviously some are better and some are not as good, I'd treat the brand position as a spectrum rather than a point. I'd say while novotel and mercure has a large overlapping part, mercure's spectrum starts from a lower, more economy end and novotel has a higher end with top, new ones pretty close to the Pullman.