Which layover?

I'd take A or B. I hate to change terminals at Heathrow. It has never taken me 1 hr and 50 minutes to switch planes at terminal 5. Going home I don't care if I miss my connection. That being said, I usually try for 2.5 hours so my checked bag goes along with me. However, If you are on BA/AA flight, if you miss your flight, they will get you on the next available flight anyway.

With the security I had to deal with on this last trip combined with my past experiences at Heathrow, I vote for 3:30

If your flights require a terminal change (BA is mainly in 5 and AA is mainly in 3) then 3:30 and hope your inbound flight is early.
If your flights are from the same terminal, then 3:30 as well.

I have never been able to get to a connection no matter where in Heathrow it might be in less than 2 hours. Then boarding time is not departure time either, you board sometimes long before the departure time especially on international flights on the larger planes which cuts into the connection time sometimes drastically. If your inbound plane is late you then have a vastly higher chance of missing your connection (no matter that you will get put on the next "available" flight onwards because that could be days if you are going at a peak travel time).

And if everything goes perfectly and you have some extra time in the terminal, that's great. You can relax a bit before boarding.

Happy travels.

Thank you all so much for your responses! They were very helpful!

The key question is how stressed will you be if you miss your flight? If the thought of rushing through the airport or missing a connection makes you feel anxious, take the 3:30 layover. If it doesn't, choose the 1:50 layover.

If it were me, I'd choose the 1 hour 50 minute connection because I hate waiting around in airports.
I've easily made connections of under 2 hours at Heathrow. I've also missed longer connections there twice, although on both occasions it was due to a delayed inbound flight and I knew that I had missed my connection before the plane even landed. On both occasions I was put on a flight a couple hours later.

With a shorter connection there is more risk that you will miss a flight. You might have to rush to your gate. The airline is responsible for getting you to your final destination. If you miss a flight, they will put you on the next flight with open seats. It could be the same day or you could end up on a flight on a different day.

Are you flying in economy? That's a pretty big factor because there are priority lines for business class travelers. I've never missed a connection through Heathrow. My luggage is not always arrive with me but that's okay with me because it is delivered by the airline. I take the same flight to Paris twice per year at least. My outbound flight always has a 1 hour and 5 minute connection through Terminal 5 (ETA from SFO is 10:40 and connecting flight is scheduled for 11:45) and the inbound has a 3 hour and 15 minute connection and I have never been close to missing the connection. I was nervous the first time and would have preferred a longer connection but now I am used to it. With no terminal change, I think 1 hour 50 minutes is fine. It has never taken me more than 40 minutes to get from the arrival gate to the departure gate.

Sad to say...yes, we will be flying in economy. My thanks again to those kind souls who responded. This is going to be the trip of a lifetime and you have helped make it even better!

Sad to say...yes, we will be flying in economy. My thanks again to those kind souls who responded. This is going to be the trip of a lifetime and you have helped make it even better!

Go to this website:

Heathrow Flight Connections

Plug in your flight information and see what the experts have to say. They will also give you step by step instructions on what to do.

Excellent advice and link. Thank you!