The two opinions are completely opposite of each other:
Most novice safari-goers automatically choose the Masai Mara instead of the Serengeti. This is not because Kenya has the most merit (it doesn't). One reason is Kenya is the world's most famous safari country and many people associate fame with being the best.
Most travelers who have visited both the Masai Mara and the Serengeti believe the latter offers a superior safari experience.
It's still worth visiting. My advice: If your first African safari trip will go to either Kenya or Tanzania but not both, then choose Tanzania. If your schedule allows both countries, definitely do both.
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