which airport?

Madrid would be preferable. And different. Go through Heathrow, and you'll always pay expensive landing fees/taxes.

Taxes and fees are not a separate, additional item. As far as money goes, just shoot for the bottom line. Heathrow will have more people, but it's bigger and can absorb them. It also has more to do while you're killing time.

MAD! With only 90 minutes, I'd be more concerned with possible flight delays, since LHR is notoriously congested and weather less cooperative. If you fly British/Iberia, there are also more flights to LIS from MAD than from LHR. Not sure if LHR is bigger than the new airport at Barajas. Certainly busier and more confusing with more terminals, but individually, the terminals in MAD are rather big, especially the newest ones (T4 and T4 Satellite).

I don't get the point of David's comment about UK airport taxes ... all I care about is the total price of my ticket. What airlines and what terminals are involved? For LHR, if you have change terminals then 90 minutes is tight. If both flights use the same terminal, then 90 minutes is ok assuming your U.S. flight is not delayed. Since the UK is not part of Schengen, you won't have to go through passport control ... just a security checkpoint. A lot of people hate Heathrow for connections, but I don't mind it. I've missed connections twice at Heathrow but never due to a short connection ... both times due to a delayed flight from the U.S. Each time, I've been put on the next flight to my destination and arrived a few hours late. If you use MAD, you will go through passport control before you go to your connecting flight. I haven't connected in Madrid so can't speak to the ease of connection.

I am not sure how to find out what terminal the flights would be out of

For Heathrow, you can check the Heathrow website to find the terminal. Check under connecting flights link.

"I am not sure how to find out what terminal the flights would be out of" First, find out who is actually operating the flight. Many flights these days are "code shares," meaning that (for instance) a United flight will also have a Lufthansa code number. For a random one I just looked up, it was United flight 62 and Lufthansa flight 7970, "both" leaving Newark at 8:20 PM for Madrid. There's only one plane, operated by United (said to be "United metal"). Often the clue, as in this case, is a flight number over 2000, meaning someone else actually operates the flight. So, to find out where a flight departs and arrives, you go to the official website of the airport, and look for the airline that is actually operating the flight. This is very important for check in; if you go to the Lufthansa terminal or desk to get this flight, you'll be redirected to the United terminal, which wastes a lot of time. Here's Madrid Barajas Airport website, listing who lands where: http://tinyurl.com/ckbh595
(Sorry, even the tiny version won't hotlink for some reason). Note that while the airport considers 4 and 4S as one terminal, they are really two terminals, each GIGANTIC. (I see that 4S isn't specified in that list separately, but it should be). And here's the airline list for Heathrow: http://www.heathrowairport.com/flight-information/which-terminal For many airline/airport combinations, the same airline can use different terminals, so you have to know the flight number or the city served.

"I don't get the point of David's comment about UK airport taxes ... all I care about is the total price of my ticket." Those taxes are the source of a substantial increase in your total fare. A substantial increase, in my perusings. As in, I'd have to really want to land in England to pay the fare difference. AND given the fact the OP is going to Lisbon, if anything goes wrong with the flight, Madrid is a better place to be. Less swimming. Besides, I read this week that MAD is in the Top Ten Most Beautiful Airports...so there ;-)

Those taxes are the source of a substantial increase in your total fare. Not necessarily. For example, out of Washington, DC, it is frequently cheaper to fly to London than any other major city in Europe. Why? Because of competition ... eight non-stop flights a day on three different airlines. Sure the taxes may be higher, but when the base fare is lower then the total just might be lower. If you are using frequent flyer miles it is a different story since you have to pay the taxes out-of-pocket. Eileen, I'd probably lean towards the change in Madrid too ... however airport taxes would not drive the choice ... the overall ticket price might.

thanks for all the answers everybody..I ended up picking a flight that had better depart and arrival times (and it was a tiny bit cheaper) and now we have a 2 hour and 30 minute layover at Heathrow but that is our only stop. The other flights I was looking at had 2 stops.

2:30 should be fine ... even if you do have to change terminals. Hope that you have a great trip.

I'm not sure about the confusion, Laura - I don't give a rat's patootie about the base fare; it's what gets charged on my credit card that counts! That's The Fare! Whether that's London ($300 + $200 taxes/fees). London ($100 + $400 taxes/fees), or Madrid ($450 + $50 taxes/fees), if London still costs $500, I'd have to flip a coin. But comparing London's $100 base fare to Madrid's $450 base fare is a non-starter. Now if London was $400 + $200, and Madrid was $500, I'll say it again: "I'd have to really want to land in England to pay the fare difference." If I really wanted to be in London and it was $600 vs Madrid's $500, I'm going to pay the extra $100 for the convenience and time (and $$$) savings. Now David's comment may have been simplistic, as in not accounting for all routes and parts of the country, but it's not untrue - it's always more money in taxes and fees to land in London. He didn't say the total ticket price would always be higher... The good news for Tracy is that now she can grab a pint ;-) Have a great trip!

Eileen, I think we are in violent agreement. I clearly misunderstood your post. My only point was that most people only care about the total price that they pay for a ticket; higher taxes don't necessarily mean that it costs more to fly via London than to fly via Madrid.

The difference in taxes & fees connecting via LHR or MAD is $10.88 The understanding that there is a large tax difference on connecting flights through the UK is not correct - mainly because there is no APD to pay, unlike if the journey started from the UK.

Hey Laura, we're cool ;-) I hear ya...!