The similarity in concept throws the differences between the two into sharp contrast. Hobb's FitzChivalry is a defender of his dying grandfather, king of a realm being assailed from outside by barbarian and inhuman raiders, and from within by the ambition of his uncle. Murphy's Belinda is the secret daughter of a thinly disguised Elizabeth I, and romps through lushly imagined courts and cities in her world's equivalents of England, France, Scotland, Italy and Russia. The one has a confined canvas that is getting steadily narrower; the other is a beginning exploration of a new world. (The religious disputes in The Queen's Bastard are nicely thought out - it is striking how rare it is to read Elizabethan-style fantasy which actually takes note of the Reformation!)
The difference carries through to how the two protagonists deal with their lovers: FitzChivalry is engaged in a difficult and discreet love affair with a maidservant, and also confused by his attachment to a young wolf; Belinda uses sex as one of her weapons, and uses it with greatly detailed enjoyment. Both are put in positions where they must choose between their political mission and their lovers, though we are frankly never in much doubt about the choice in either case.
I ended up liking both books. Royal Assassin seemed to me to have quite a slow start, but gets going about a quarter of the way in. The Queen's Bastard is a bit more breathless - the change of tense to present when we are getting Belinda's point of view is part of that - and it's also a very sexy departure from Catie's usual style. I suspect that people who like the one will like the other.