Lisbon to Porto

Take the train much easier. About 2 1/2 hours

Yes, flying from Lisbon to Porto would be silly. Take the train, buy your tickets at www.cp.pt

I took the train from Lisbon to Porto and it's very easy. I haven't checked flights on that route, but flying will be much longer once all the travel to & from airports, check-in, security, waiting times, etc. are added, more stressful, more of a hassle and possibly more expensive once all the baggage and other charges are added.

If you are indeed an "Easy Traveler", take the train!

Take the train. Inexpensive, quick and comfy.
When you book your tickets to Porto São Bento station (most central station in the city), you'll have to stop at Campanha station first for a quick change to the commuter-style connection to São Bento. It's included in your ticket.

To answer your question, TAP Air Portugal would be your best choice with many non-stop flights a day.

But the train is a nice way to go :)

TAP is not only your best choice if you want to fly between the cities, it's your only choice.

When you see two large cities close to each other with no low cost airline flying between them, it usually means that they can't compete with the train. Which certainly is the case for Lisbon to Porto. Buy your tickets in advance and the Alfa Pendular (high speed train) between the cities can cost as little as €12 per person.

It seems taking the train from Lisbon to Porto is the way to go. The reason I considered flying is the fact we will be taking 2 full size (50 lb each) suitcases on our trip. Unfortunately, as many times as we have tried to lighten our load, it does not work. We are senior travelers as well. As long as there are staff at hotels and drivers that can handle the suitcases, we are good to go. The fact we would have to carry our luggage on and off the train and then store it out of sight concerns me. Are there train employees that help people with their luggage? Is it safe to store it out of sight?

Thank you to everyone that has taken the time to answer my questions and concerns.

CORRECTION: I meant one full size suitcase each. Two total.

Here's a link to Seat 61 which is a good resource regarding trains and has details regarding this route.
https://www.seat61.com/trains-and-routes/lisbon-to-porto-by-train.htm

I don't have an answer regarding if there is assistance available regarding your luggage. I know relying on the kindness of strangers probably is not the best answer. You could contact the Portuguese railway and ask this question. I've emailed them before and found them responsive. https://www.cp.pt/passageiros/en/Contacts
There is a section under Special Needs Customers where a request can be made that might work for you. https://www.cp.pt/passageiros/en/how-to-travel/Special-needs-customers

Unfortunately, there was not anyone at the train to help us with our bags. We both had small carry-on size bags and managed on our own. I can understand wanting to fly for that reason, especially with larger suitcases.
Happy to report I've stored bags out of sight on trains all over Europe and never had a worry or an issue.
As far as the kindness of strangers, in Portugal this past October, we took several trains and never was help offered. I had a cane, a crossbody bag, and my carry-on suitcase. It would literally take me a full minute to board or disembark the train, one slow step at a time, and had younger people stand there and just watch. Not one offer for help. This lack of courtesy was quite disappointing.
Sadly, I saw this on the Lisbon and Porto metro trains as well. Young people sitting while I with a bag and a cane, or other older people with shopping bags standing while the young people sat, not offering a seat.
Conversely, in Italy this past November, two other senior friends and I were offered help at the train station numerous times.

Unfortunately, as many times as we have tried to lighten our load, it
does not work.

Then you probably should try harder. Travelling with luggage too heavy for you to handle sounds like a really bad idea.

Are there train employees that help people with their luggage?

No, you have to carry your bags yourself.

Is it safe to store it out of sight?

I don't worry that much about it. I keep an eye on it until the train departs, after that I rarely worry.

Are you returning to Lisbon after trip to Porto? This past fall we did this, and left primary piece of luggage at hotel where we could pick up again before moving on.....small bag for two nights to Porto on train.