Even if he has his flaws I would defenitely choose Guardiola over him if I'm hiring for a big ambitious club (and they would too, hence why he is replacing him instead of competing with him for gigs, EPL is his preferred destination after all). Ancelotti tends to get too comfortable and regress too quickly in following seasons with his clubs, issues of consistency in longer term (not so flexible with squads, tends to become too chummy and overplay the same players/11). Was pretty damn unacceptably average for long time with Milan, and tho they were trigger happy, its not suprising he didnt last more then 2 seasons with either Chelsea or Real. He lacks the ability to keep a team hungry and motivated, and to reinvent himself if things become in a rut. His type of possession/midfield focued football and mentality is suited well for the cup/tourno format that is CL, and its the main reason of his stellar reputation.
I repeat, 4 league titles 18 years is not good enough for the best (not counting from Parma, but from stint with us). Conte tho is also blessed with good timing when he came to us (tho in comparison Ancelotti almost allways just coached the best players in each league), atleast has 3 league titles out of 3 in his first attempt with a top club.