You should be fine (assuming your inbound from Harare is on time). Once you disembark you'll have to clear DXB airport security. This is normally pretty quick and it's centrally located. This means that although the terminal is huge, you won't be more than a 10-15 minute walk away from your departure gate. Your boarding card will tell you to be at the gate an hour before departure but don't let this stress you out. Boarding typically commences 30 minutes before departure and the gate won't close until 15 minutes before so you'll have about an hour to play with. It might help if you select a seat near the front of the aircraft so that you can disembark swiftly...
Personally, I'd go for the quick connection which allows you to fly into Gatwick. If you end up missing it, then this would be the fault of the airline and they would be obliged to put you on the next available flight. Chances are you'd end up on the Heathrow flight anyway.
Last edited by Anansis; 29th Sep 2012 at 20:06.