flights to US from LIsbon

Depending on where you live, you could fly into Newark instead. 21 flights per week on two airlines, according to directflights.com

If you're willing to consider adding a connection into the travel schedule, you
could just book a one-way flight from Lisbon to some more major European
gateway city, and book the transatlantic flights as an open-jaw arrangement
(eg, PHL to MAD, LIS to CDG, and CDG to PHL).

If you did something like that, the fact that the one-way would be on its own
ticket means you would most likely want to have a night in the gateway city
prior to flying back, so any irrops doesn't affect your flight back to PHL.

Traveling by bus or train from Lisbon to Madrid is much harder than it seems
it should be, thus the input to fly to any city that has nonstop flights home.

The fact you have a direct flight from Philadelphia to Lisbon is quite lucky, not an option for my home airport (I am going in November). The cost/benefit of a direct flight is up to you of course, but I'm always willing to pay a bit more for efficient travel. Put a fare tracker on the flight now and watch how the prices adjust, if you see something you like book it immediately.

TAP Portugal flies direct to several American cities. I’ve flown them many times, both within Europe and back and forth to U.S. They are my preferred airline, especially within Europe. But, I’ve found business seats very reasonably to the U.S., too. I would not hesitate to fly with them in the future.

You may find bad reviews for the airline, but it is for changing flights, etc. If you are certain of your flight, you will be happy with the service, planes, flight attendants, etc.

Madrid or anywhere in Europe will give you more options. Unnecessary to focus on Madrid only as, once you are on a plane, it doesn't matter if you fly 15-30 more minutes to a city with better connections.

Just get on a flight search engine like Google, do a multi-city search from the drop down where it says round trip and see what comes out that is within your price range and suitable to your schedule.

Do a search LIS-PHL on Kayak or Google Flights for your dates and see the options. If one layover gives you a much better price, take it. There is no obligation to take a non-stop, although it is generally more convenient.

TAP and United are Star Alliance partners, so most options will be through EWR or IAD, which are United hubs on the east coast. TAP also has several non stop flights to Boston, given the large Portuguese population in Boston.

. Is it better to return to Madrid by plane or train for more direct flight options?

A train routing from Lisbon to Madrid is 10+ hrs., with multiple changes; a direct flight is less than 4 hrs.
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To me, the obvious choice is to fly.

If the direct LIS-PHL flights are 'too pricey", then the choice seems to be a one stop option or going back to Madrid for a better selection of direct flights.

I would choose doing a one stop from Lisbon and just deal with getting to airport once, rather than the twice that flying to Madrid from Lisbon and then back to the airport again for the flight home.

If it is indeed that you wish to go from Philly to Lisbon, may I suggest being patient for a spell? Philly is my home airport, and almost always on American. My experience the last several years is that there is a sizable price drop on American in mid to late January. I also note that the open-jaw option to, e.g., return from Madrid via American is almost the same price, and that direct on TAP for Newark to Lisbon RT is over 20% less. I cannot tell where you are and how convenient or inconvenient going through Newark would be, for me living in Philly I do not want to have to be coming home from Newark after a flight from Europe. Unfortunately, we are very limited in airline options here in Philly airport.

Thank you for your advice. We were able to get a fair price from Lisbon to London - then to Philadelphia.
I decided to book it due to rising fuel costs and the flights and prices were decent, considering that
world travel is currently affected by the state of confusion in the US.