There have been a lot of comparisons between the ARC-170 and X-wing, indeed the two were built by the same company and several members of the ARC-170 design team participated in the design of the X-wing.
The ARC-170 was a response to the need of the Republic's Navy for a heavy hitting long range fighter to engage the enemy and escort long range strike fighters like the Y-wing, something that the more common Alpha-3 Nimbus and V-19 Torrent could not provide with their limited range. The Z-95 which served as the initial basis for the ARC-170 could provide long range escort, but it was considered to be too lightly armed for the task.
The X-wing was born in the post-Clone Wars era as a follow on for the aging ARC-170 still in limited use by the Galactic Empire and was a more direct descendant of the Z-95. However members of its design team had anti-Imperial sympathies and eventually defected with the initial prototypes and design plans and the X-wing became the primary fighter for the Rebel Alliance once it was available in sufficient numbers to replace older designs.
The two fighters share many design features, s-foils to help extend the craft's shields further out from the hull of the fighter and diffuse the heat build up from weapons fire. In addition they both carried hyperdrives and attendant astromech droids to enable multiple jumps and thereby independent long range missions. Their armament was also very similar, comprised of laser cannons and proton torpedoes. We'll address these various systems now and see how the two compare.
Speed and Maneuverability
The ARC-170 and X-wing have comparable speed characteristics with only a slight advantage going to the X-wing which can attain slightly higher top speed though in a dogfight this would mean very little. However there is a larger difference in acceleration where the X-wing has a massive advantage, the two craft might have a comparable top speed but the X-wing is able to reach that top speed much faster than the older ARC-170 is. In maneuverability too the X-wing has a clear advantage, able to maneuver far better than the larger ARC-170. In a chase, the X-wing could put on an initial burst of speed to get away from the ARC-170 and maintain the distance between them, however without a head start it would not be able to catch the ARC-170. In a dogfight the X-wing would be able to out turn the larger heavy fighter.
Shields and Weapons
At first glance the X-wing has an advantage with four forward-firing laser cannons and six proton torpedoes divided between two launchers to the ARC-170's two forward-firing lasers and the same number of launchers and torpedoes. However this is deceptive. While the X-wing mounts more head-to-head weapons, these weapons are smaller than those of the ARC-170 whose forward firing cannons pack a much bigger punch. As a follow on to the above mentioned superior maneuverability enjoyed by the X-wing, the ARC-170 also has a rear facing twin laser turret able to cover the blind spot that the X-wing suffers from. The proton torpedoes are comparable across both platforms. In shields however there is a larger difference. The X-wing has a much weaker shield system than the ARC-170 which was designed to take multiple hits and keep the starfighter in the fight. In fact the ARC-170 has a shield system more akin to that of the Y-wing than the X-wing.
Logistics and Crew
Here is perhaps the biggest difference between the two starfighters. The ARC-170 requires a crew of three plus one astromech to be effective. The X-wing needs only a pilot and astromech droid. On top of this, the X-wing has a larger stores capacity, meaning it can stay out longer than the ARC-170. Both craft are able to mount additional stores in external pods, but with the additional crew requirements of the ARC-170 these can only extend it by so much. All of this means that it is cheaper to field an X-wing than it is to field an ARC-170, which is probably why during the Galactic Civil War the Rebel Alliance deployed what ARC-170s it had (which it had the capacity to build) in smaller numbers and then only to elite squadrons whereas the X-wing became the staple of its fighter force.
It should be noted that the ARC-170 has one major advantage over the X-wing. While both craft mount the standard flare and chaff countermeasure systems, the ARC-170 also carries an extensive electronic warfare suite which enables it to jam enemy sensors and disrupt their communications. The X-wing has no such capability.
At the end of the day it is important to remember that while these two craft sprung from similar lineage, they were designed for two different roles.
The X-wing was designed as a general purpose starfighter to be mass produced that could accomplish a wide variety of missions while specializing in no one particular role.
The ARC-170 was built as a long range heavy fighter to escort strike fighters and provide a heavy hitting punch to backup its much more numerous light fighter counterparts.