I think Master of the Legion + Centurion is the better tax. The MotL unlocks 3+1 Elites, which is about as many as you'll ever need, plus +1 Reaction, while the Centurion brings, if you're using terminators, Land raiders, and if not, 4 tanks, plus whatever the rest of your list will be.
And it's fluffy. Remember how every game of 7E you played you had your chapter master and another captain? Praetor + Centurion is the same. Your legion only has some 20 Praetors...and he's somehow always in your list.