Most foreigners find it easier to learn Brazilian Portuguese than the European version, for example.
This is because Brazilian Portuguese is simpler and they speak more slowly.
Mutual or partially mutual intelligibility has nothing to do with languages being "easier" or "more sound complete.
I could not find the right word to describe what makes a language harder to learn: all I know is the closer the new one is to the languages you know, be it similar words or grammar, the easier it is to learn (since you can use a lot of the knowledge from the languages you already know). Also if you have to learn new sounds it makes it harder to communicate in that language because you "butcher" it.
Portuguese is the 5th most spoken language in the world, the vast majority of its speakers (Angola, Mozambique, Guinea, etc.) would not find it so easy to transition to Spanish. Portuguese and Brazilian people do because... their countries neighbor Spanish-speaking countries.
Neighboring countries help, unless you are from a germanic/slavic speaking country and your neighbour is Finno-ugric. Or a Latin language and a germanic language. The closeness of the languages is what is important, not whether they are from a neighbouring country. I come from a part of the Netherlands close to the German border, it was perfectly possible to speak simple Dutch to a German and the other way around However, the further inland in Germany the worse it performed (less exposure/dialect). If I try to speak simple Dutch to a French speaker (forget wallonia for this example) I will get blank eyes.
Oh, and you know that the vast majority of Portuguese speaking are in fact living in Brasil?
When it comes to languages, politics and geopolitics often offer a better explanation than linguistics. Visit any high school in Portugal and you'll verify that linguistic proximity doesn't make it easier to learn French; kids get better grades in English or Mandarin
This does not provide any point whether a language is easier to learn only why they are chosen. To start: grades do not imply how easy it is to learn, maybe the effort put in goes down to get the same grade. Second, that people prefer English over French could be because much more people speak English than French. Finally, for most people there is almost no return by speaking an additional close language. Often you can use English and if that does not work you can just use your own language but speak slowly and avoid difficult words.
When in high school do you already know that you want to live in another country or that it is needed for your future work? And if so, do you know which country? I do not think many high schoolers know the answer to these questions and then you get the geopolitical choices.