Hi – if you have the budget, I would lean to the hotel option – I love the Sofitel but LHR has a few other options as well given that you have time to change terminals. I would allow about an hour to clear immigration unless you are flying business class with a fast pass or have access to gate entry. Then allow two hours to clear security and get to your gate on the return. So of your 9 hour layover it will most likely be about six hours. The lounges at LHR T5 are almost always crowded unless in the B gates. There is no sleeping option unless you have access to the Concorde Room in T5.
Hope this helps!

nice that Dubai provides (like Changi) space for layover folks to relax and catch some sleep rather than be stuck on the airport floor or at the gate in uncomfortable chairs. Dubai now offers daily use of their lounges for a fee – depends on your willingness to pay for some comfort as the prices are high for 4 hours

Sandra – for the flights that arrive at 6:30 am, i go right to sleep at the hotel (booking the night before so I can just check in and go to sleep), I generally sleep until mid-day, then go out for late lunch and a walk to clear my head. Have dinner at 7ish and then I’m all good – difference is that I’ve been doing that routine for 10 years with work. For the later flights, I stay up all day until I’m about to drop. I keep on a heavy routine of food, snacks (I bring my own) and water. By 8 pm, I’m falling asleep and then set for my 6 am wake up. Hope this helps