They were extremely similar in some ways, but complete opposites in others. The similarities is in the smooth technique and in the spatial and temporal awareness.
Perhaps you can make the point that a player able to play in this "controlled" way while at the same time regularly be decisive with crazy dribbles and creative assists etc (like Iniesta) is more unique than someone who barely controls the game by keeping the ball at all times and play the correct, safe pass 999/1000 times (like Xavi). But I feel, as someone already pointed out, that you could replace Iniesta with Silva without noticing much difference, but if Xavi was to be replaced (by whom?) the entire team would almost fall apart.
I felt Xavi went on the field just to prove that the easy solution is enough, that you do not need to look for the decisive action all the time (like Magnus Carlsen said about Bobby Fisher - he proved that 'easy' chess is enough). He was an extremist, and influenced not only his team but the entire footballing world as soon as he got the chance to play his way to the fullest. Of course he was extremely gifted, but as I have written a lot of times here, it was only after the year he turned 28 anyone ever thought about him entering into some GOAT discussion. Iniesta was probably more gifted, but less ideological and less unique, more of an artist kind of player the likes we've seen a lot of. Xavi was lucky to finally get permission to play his type of way fully, Iniesta was lucky to be in the same team as Xavi all the time.
