It's a bit like comparing apples with oranges, the two are in very, very different categories.
In terms of the specific questions,
From Cocobay, you have Little Ffryes beach right in front of you, and easy acess to Ffryes and Valley Church Beaches. In spite of the good reviews (almost all from cruise ship day visitors) Valley Church is no great shakes - there's a very average beach bar/restaurant, the water is not crystal clear because of the minute sand particles suspended in it, but is a great colour. The bed/parasol vendors are tiresome, and the beach has some very shelly areas.
Ffryes is lovely, as is Darkwood (next beach along to the south)
Service is (unsurprisingly, given the price) better at Hermitage. It's very, very good at Cocobay, however.
The beach at Hermitage can be disappointing for some visitors. It's not smooth fine sand, and has many small shells, which can be uncomfortable underfoot in some places.
The biggest difference is in the food - obviously it's better overall at Hermitage Bay. However, with the difference in prices, you could eat at some of the other really excellent restaurants in the area (including the fabulous Sheer Rocks, which is right on premises at Cocobay) for every meal of your stay and still come out ahead!
Hope that's some help!