Both McLaren and Ferrari have heavily upgraded their cars over the past two races as the 2024 grid continues to converge.
McLaren has looked particularly competitive in recent weeks, with Lando Norris taking his maiden win in Miami while challenging Red Bull's Max Verstappen until the final lap in Imola.
But with Ferrari also within one or two tenths, Marko is expecting the Scuderia to be an even bigger threat for pole this weekend, with qualifying all but decisive around the principality's narrow streets.
Over the past 20 years the polesitter has gone on to win the race on 13 occasions.
Asked if Monaco will be Red Bull's stiffest challenge so far, Marko replied: "Yes. We have to take on this challenge.
"But I think the Ferrari will be more of a challenge in Monte Carlo than McLaren, because it's all about qualifying. We have to take on this challenge, definitely."
Ferrari's Charles Leclerc is yet to snatch his first home win but has two poles to his name in 2021 and 2022.
But team principal Horner is leaning more towards McLaren due to its searing pace in Imola, where a tow for Verstappen from Nico Hulkenberg helped him overcome Oscar Piastri and Norris to keep his clean sweep of 2024 pole runs intact.
"I think you have got to say McLaren. They have a very fast car at the moment," Horner said.
"Piastri looked quick at certain moments, Lando was very quick at the end of the race there.
"But Ferrari are in the noise with them as well, so that is another four cars we are up against, which is great for you guys!
"The race is won on Saturday, fundamentally, so having one-lap performance and having a very strong front-end is going to be very important."
McLaren has gone some way towards making its car a better all-rounder that can be competitive across different cornering speeds, and therefore Horner expects the Woking-based team to be in the hunt everywhere.
"Over the last couple of races they have been the main competitor and they have definitely added performance to their car," he added. "The philosophy is very similar to that of our own. We expect them to be competitive at all circuits."
Additional reporting by Erwin Jaeggi and Jonathan Noble
