Staying at a camp inside the park means your safari starts almost immediately when you leave the camp, whereas outside the park you can spend up to 30 mins getting inside. Makes a big diff at 630 am!
Masai Mara national reserve is one of the best place on the planet to see big cats including lions, leopards and Cheetahs. I have been there for 6 times for big cats reseach and conservation. Masai Ma