Heathrow Shuttle will take the two of us from Heathrow to the Marriott Marble Arch for £28. This seems like a very hassle free and reasonable way to go. I could not find in your book any mention of this option. Is there a reason you do not suggest it? Thank you!
Firstly Rick Steves doesn't read this board - it's purely a self-help thing. As to why he doesn't recommend that specific shuttle service - no idea. It's more expensive than the underground and you might spend some time sitting in traffic.
The shuttle would cost less than a combination Heathrow Express + Taxi, and you would have to handle your luggage less. I haven't taken airport shuttles in Europe; I'm always concerned that it would be difficult to meet up with my driver. And, what if the plane arrives late? If that is 28 GBP for two, it sounds like a deal.
Perhaps Heathrow Shuttle is new, but £28 from Heathrow to your hotel is pretty good. I wonder if it's like SuperShuttle in the U.S.? Ask them how many other passengers they take onboard and how many stops they might make before they drop you off.
Heathrow Shuttle is not a new service; it has been around for some years. Have no personal experience. Internet reviews seem variable.
Its all in what your personal tolerance is. If you are really bothered about carrying luggage and dealing with it, a shuttle might not be a bad way to go. Personally, I tend to NOT use shuttles because they have to sit in traffic, and because air travel is anything but predictable these days. I dont want to worry about missing a shuttle just because I got delayed due to a mechanical, weather, or air traffic flying across the pond. Having said all of the above, 28GBP does sound like a decent deal for two people. Did they give you a rough estimate on how long the shuttle would take?