#2 Style of play
The most obvious similarities between the two midfielders are their work rate. Both Modric and Guti are sort of like the 'gears' that runs/ran the machinery of Real Madrid. Quick, agile, and genius, the two are the ideal playmakers when it comes to the midfield.
While Guti played mostly as an attacking midfielder, Modric in recent times has developed into a #10 as well, as was evident with his time with the Croatian national team.
However, their positions aside, they have distinct differences in terms of their style of play. Modric is a typical central midfielder, a midfielder who controls the flow of the game and takes charge of making sure his team does not frail, building the game from the back, playing box to box, as well as using pace and long-range passes. He is regarded as the master of the 'pre-assist'.
Guti on the other hand, was responsible for beating the defence in order to create the attacking opportunity required in order to score goals. His excellent vision as an attacking midfielder made him one of the players with the most number of assists in multiple seasons.
Unlike Modric, the Spaniard would usually situate himself in the midst of the action, waiting for the right opportunity. He was also known for doing the unexpected, such as accurately delivering a pass using the back-heel.
Overall, Modric looks like a more 'complete' midfielder, as he has the ability to play in multiple positions.