Which one is better? Bench press with a cambered bar or bench press with a standard bar?
Cambered bar bench presses are less effective for the chest muscles and triceps than standard bar bench presses. The cambered bar bench press, on the other hand, is a very good exercise for the delts. Polish sports scientist Michal Krzysztofik discovered this in a human study that appeared in Frontiers in Physiology.Study
Many strength athletes who use the cambered bar do so because this it enbales them to do heavy squats while protecting their shoulders. But of course you can also use the cambered bar for bench press.

Krzysztofik had 12 male students, all of whom had been weight training for several years, bench press several times with a regular bar and cambered bar. He had attached electrodes on the men's skin that measured how hard the muscles underneath had to work.

Bench presses with the cambered bar generated more electrical activity in the delts than bench presses with the standard bar. On the other hand, bench presses with a standard bar provided more electrical activity in the pecs and triceps.

The heavier the load, the greater the differences between the two versions of the bench press.
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"The cambered bar was more effective in activating the anterior deltoid muscle than the standard bar during the bench press exercise, which seems particularly important for sport disciplines that engage the anterior deltoid muscle during specific upper-body movements such as striking, throwing, punching, and hitting", the researchers summarize their findings."On the other hand, the standard bar bench press more efficiently activates pectoralis major and triceps brachii long head in comparison with the cambered bar bench press."
"Thus, the cambered bar may be an effective modification of the bench press exercise to additionally overload and develop the anterior deltoid muscle, while standard bar bench press may be beneficial for those seeking to maximize the involvement of the pectoralis major and the triceps brachii."
"However, athletes with limited shoulder mobility and/or past shoulder injuries should use the cambered bar with caution, progressing slowly toward improving the range of motion in this exercise."
Front Physiol. 2020 Jul 16;11:875.