I’d never before even considered this question, so I want to make sure to include my references so that you’re aware. According to Barbend both are equally important (well not important but they aren’t both completely the same). I won’t try to reword the content placed in this article (the one I’m going to link to) but what I will do is say that both movements can improve the strength of the posterior chain but in different ways. And not only that but they both have different benefits. For a more in-depth answer:

For a more in-depth answer:

The trap bar squat

The trap bar squat is a movement that follows a deadlifting pattern but offers a degree of knee flexion and quad involvement, the majority of the emphasis is on the back and glute development, however.

The back squat

In this article, the high bar variation is what will be discussed as the shin and torso angles are similar to one another.

The distinct difference between the back squat and trap bar squat

Below one will find 3 aspects that athletes and coaches should consider when they're determining which of these movements is best for their program/athlete's goals.

Posterior chain dominance

The posterior chain can be defined as the back, hamstrings, and glutes; as well as the synchronization of these muscles to produce powerful and explosive ankle, hip and knee extension. These things are critical for almost every athletic feat.

Both movements can develop the hips and glutes into powerful muscles, in different ways. The trap bar squat relies very heavily on upper/middle back strength and glutes, while the back squat relies on the quads and back. When the movements are programmed efficiently, they can help produce powerful athletes.

Leg development

The trap bar squat follows more a deadlift pattern than it does a squat. The movement lacks a high degree of hip and knee flexion if it's compared to the back squat, which minimizes the movement's ability to develop the hamstrings and quads during the full movement. Individuals who rely on the full range of motion force production would be hindering their leg development performance, and resilience to injury if they omit the back squat in place of the trap bar squat.

Load bearing benefits

The trap bar does allow for some spinal loading, but the stress is placed on the lumbar spine less than it would be with either high bar or low bar squats. For some, this could be intended to lower lower back stress, but for some healthy athletes, the higher level of spinal loading could lead to higher long term leg development and resilience to injury.

When loading is in mind, almost every strength and power athlete should be able to brace, carry, pull, and squat, which makes both of these movements very beneficial. The squat does, however, offer slightly more benefit in this area.

Some people consider the trap bar squat an actual squat, but it should really be categorized as a trap bar deadlift because of the lack of knee flexion in the entire range of motion. Even though the torso and shin angles could be similar to the back squat at some times, there is no way that the trap bar squat is a replacement for either front or back squatting, since squatting is one of the most total body strength, transferrable, and functional movements the human body can do. If only one movement could be suggested it would be the back squat, mainly because its universal application to almost every athletic activity.