Nothing like a good old LHR hotel stay discussion.
The OP mentions Hilton T4 and Sofitel T5. Given your connection, either one is fine in my opinion. If you had a shorter connection, Sofitel would be preferable but the Hilton at T4 is just fine with this connection time. See the Hilton T4 thread for the lengthy discussion on best way to get there.
Your profile lists Hilton Diamond, so if you want to stick with that program. I would consider the Hilton Garden Inn, can get there via bus or tube, see that thread on details. It will be cheaper and a cozier and cheaper option. If you want to get away from the airport, which I would likely do with this particular connection. I would consider Paddington Hilton. You will get there very quickly and you have a much more interesting area than the airport. I typically only to the Paddington option if I can get it with a decent points option.
As others have mentioned, with this connection you could really do about any property at LHR. I would personally prefer my bags to be checked through to IAD and get my boarding pass in FRA. If you have fast track access that would help particularly at immigration.