I've stayed at the Ren Heathrow & used both Hoppa Hoppa (a charge) & the bus (free). I had to wait forever at Heathrow for the HH; the pro is that it dropped me off at the entrance to the Ren. I used the bus to go to/from Heathrow 3 days in a row & caught Picadilly line into town (cheaper than HExpress although more stops); no charge (and you can take your luggage on as you would w/ the HH); con is that it drops you across the street from the entrance so if you do have luggage you'll have to shlep it across to the hotel entrance. I found the Ren Hth to be fine. Marriott a bit more modern & just down the street. The bus will also drop you off at the Marriott Hth.