For me "best" in this case is a combination of convenience and value, assuming one has to catch a United flight in the morning.

I asked the question because it's not clear to me that either of the in-terminal hotels (Hilton in T5 and Sofitel in T4) is convenient to T2 (United). At most airports I like to stay in-terminal for convenience but Heathrow's layout may make an off-site property a better choice. LHR is the rare airport where--from T2--an off-site hotel may be quicker to get to than the on-site hotels in other terminals.

Of course, even if the in-terminal choice is the most convenient, I'm not necessarily willing to pay double or triple the rate at an off-site property, but I' can make that judgment after determining which is most convenient.