In economy, I would definitely take (and have taken) the 9am departure. Stay awake for the whole flight (as I would normally between 9am and 10pm) and arrive into LHR at 3pm , means you get to check in to hotel and shower on arrival, and after that it's only about 4-5 hrs to bed and I find that better for jet lag than having to stay awake the whole day. (although I do note time is not a factor for me - I can spend a good chunk of the flight working more productively than sitting in office, thereby avoiding the need to take that day as leave).

As for Suites, we were faced with this very decision when looking at some flights on the infamous RGN-YUL mistake fares (which unfortunately were cancelled by the issuing airline ), but it didn't take us long to choose the 9am flight. We felt it would give more of a chance to enjoy the first class experience than an overnight flight. Although I realise you can't spend the whole flight eating and drinking - in terms of that - I just don't enjoy eating big meals when my body clock says its 2am, whereas I know I would during the period 9am-10pm.

Overall, if you're a regular flyer in long haul F cabins, I can see the late night departure having merit. If you seldom fly (or have never flown) long haul in F then I think the novelty factor, ie experiencing F service, would point to the 9am flight.