Push pull legs and upper lower are both effective workout splits but the main difference is how you divide your training days in push pull legs you train pushing muscles together pulling muscles together and legs separately while upper lower divides workouts into upper body and lower body sessions both can help you build muscle depending on your training schedule and recovery.
I think upper lower is better for beginners because it is easier to manage and you can train your entire upper body in one session and your lower body in another session this makes it easier to learn the basic exercises and build a consistent routine without spending too many days in the gym.
Push pull legs is great if you enjoy going to the gym more frequently because it separates chest shoulders and triceps from back and biceps and gives your legs their own workout day it allows you to focus on specific muscle groups and include more exercises for each area when your weekly schedule allows enough training days.
The biggest difference is training frequency with a four day upper lower split you can usually train your upper body twice and your lower body twice each week with a six day push pull legs routine you can repeat the cycle twice weekly but you need to manage training volume and recovery carefully.
I prefer upper lower because I can get a good workout in four days a week and still have enough time for work and other activities I train my upper body on Monday and Thursday and my lower body on Tuesday and Friday which gives me a simple schedule that is easier to follow consistently.
Push pull legs works well for people who enjoy spending more time in the gym because each workout has a specific purpose you can focus on pushing movements one day pulling movements another day and leg exercises on the third day but missing one session can make your weekly schedule harder to manage if you do not plan ahead.
If your main goal is building muscle both workout splits can work equally well when your exercise selection training volume effort and recovery are appropriate the best choice is not necessarily the split with more training days but the one that allows you to train consistently and progressively improve your performance.
One thing people forget is that push pull legs does not automatically build more muscle just because you train six days a week if you perform too many sets and do not recover properly your performance may suffer a well planned four day upper lower routine can produce excellent results with less time spent in the gym.
I like push pull legs because I can concentrate on one group of movements without rushing through every upper body exercise in the same workout chest and triceps receive attention on push day while back and biceps get their own session this makes it easier to organize exercise order and focus on individual goals.
If you want to build muscle but only have three or four days available I would lean toward upper lower because it is easier to distribute your weekly training across those days you can still use isolation exercises for biceps triceps shoulders and calves without needing an additional workout day for every muscle group.