I've stayed at the Sheraton Heathrow and the Sheraton Skyline quite a bit.
The Sheraton Skyline has more food options in the area and a delicious Indian restaurant in the hotel. The rooms are pretty old. Lounge is ok. There is a pool. More convenient for non-T5 terminals.
The Sheraton Heathrow has been renovated and the rooms are cozy, although on the small side. Food options are pretty limited (McDonald's across the street), and I didn't really enjoy the restaurant. The gym is meh. Convenient for T5 though.