I've got a flight mid-August from T3 and hotel prices for the night before are:
T2/T3 PI £91.50
T5 PI £76
Bath Road HI £115
Bath Road Staybridge £148
HGI Hatton Cross £132
HGI T2/T3 £210
Travelodge Hounslow £74.99
Thistle T5 £94+
Sofitel £190+
Some disparity with these prices. As I said, don't feel the (£110) premium for the Sofitel is worth it over the T5 PI.
I'm more swayed to drive down overnight and park somewhere around 6am, then get the tube to T3 for my mid-morning flight.