It's true that T-Mobile has an international plan for free that you can use, but it depends on the TM plan that you have. I don't think it's offered to all of T-Mobile customers. That said, you can pay for an international plan if you aren't entitled to a free one.
However, in the past I have used Airalo, and sometimes it's a good idea to have an eSIM in addition to your international plan just in case. For example, when I was in London last year, I had a lot of trouble with the carrier that was providing service for T-Mobile and was stuck without service for a lot of my stay. It was extremely frustrating, and I wish I would have just gone ahead and gotten an eSIM that I could have used to supplement the plan.
With regards to your question about which package is a better value, it really depends on what you need. If I was traveling to Schengen countries and the UK, I would get the EU and UK package. Otherwise, you might get stuck. For example, I had an Airalo eSIM when I went to Germany back in 2022. I bought just the Germany package; however, while I was there, I decided to head to Copenhagen for several days. Unfortunately, my Germany plan wouldn't work there, so I had to buy an extra five days of service for Denmark with Airalo. It wasn't expensive at all, and it worked fine, but I'm just saying it's probably a better value to get the package so you don't have to change eSims when you switch countries.