To me the bad parts of this trip are the change at Hammersmith or wherever to the District line - crowded platform/crowded train with luggage AND when you finally get to Victoria Station which always seems to be a crush as you try to exit up the stairs to get out of the station. At that point both the District/Circle lines and the Victoria line dump together in the entrance hall to exit. Getting on at the airport is no problem - lots of empty space there!
I would go for the coach. I will say that I've never gone from the Heathrow Coach station to the Heathrow Underground station but it feels like the Coach station is kind of out on it's own at the end of a corridor. That may be a mis-interpretation as I've only done the coach once and I've done the Tube/train several times. BUT, I don't think it will be efficient to check on the coach times and if too long to double back to the Tube Station if that is your intent.
If you do take the Coach, when you head back to the rail station, stay on the sidewalk on the coach station side of Buckingham Palace Road and cross the street by the Starbucks/Pret a Manger. The sidewalk on the other side of the road is always congested at that corner. This still puts you in to some crowd as there are tour buses and regular bus stops lined along this side of Victoria Station but not as bad as trying to thread thru the corner crossing of Elizabeth Street. I hope that makes sense?
It may be a moot point anyway as it may take longer to get thru Immigration than an hour.