I have flown AF in J from CDG-HKG with the new business class seats and several times been on EK in F.

Food on Emirates F is far better to AF J in my opinion.

Seat comfort depends on what configuration your EK 773ER flight has. However either way you'll have plenty of room up there in the forward part of the cabin whilst on AF J you'll still be together with a relatively large group of people.

IF flying EK you'll have a car pick you up from your home/hotel in JNB and also on arrival in LON. I do not think AF offers this.

Spending a day in Dubai can be interesting, if it's your first time here it might be worth it.