Grad school decisions: move to a different country to pursue PhD, or stay with S/O and do PhD together?

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I have been agonizing over a decision regarding where I should pursue my PhD in theoretical chemistry. My choices are as follows: university A (US) has the best department in the world for my area of research, and my potential PI would be the perfect fit for my interests and career goals. University B (my undergraduate alma mater in the UK) also has a very good department (ranked 5th in the world), although the advisor fit would be less than ideal, since their group is mostly experimental, with only a few people working in theory. The group at university B is very large (~70 people), so the advisor is able to spend very little time with each PhD student. Both potential PIs are very renowned in their respective fields (h indexes above 60).

Here comes the kicker: I am in a very committed 4-year relationship with my S/O, who is going to start a PhD at university B. He is supportive of me pursuing a PhD with my dream advisor at university A, which would involve starting a 5-year (very) long distance relationship. However, I can't help but feel selfish for putting us both through LDR hell.

What would you do in this situation? Since both of us want to stay in academia long-term, I know that at some point in our relationship we will likely have to face similar choices again, so how does an academic couple balance the two-body problem for every new position they apply for? Any personal anecdotes or ways of solving this dilemma are very much appreciated. Thank you!