As I understand the two phrases, "it would be best to do X" treats doing X as one of the optional courses of action that a person, couple, or other entity may elect to pursue. Other options might be to do U ("see each other all the time"), to do V ("see each other more often than we do now"), to do W ("see each other the same amount as before"), to do Y ("see each other less often"), and to do Z ("see each other one more time and that's it"). The person who says that doing X is best means that it is superior to options U, V, W, Y, and Z, as well as any to others that may have suggested themselves during deliberations over the proper course to take.
In contrast, "It would be for the best to do X" focuses on the general betterment of all concerned, taken on balance. Something that is "for the best" is the most beneficial course—the one that a person taking a dispassionate, utilitarian view of the situation would endorse as optimum. Realistically, some options (the pit or the pendulum, say) don't have a "for the best" alternative; the decision really comes down to choosing your poison—in which case the choice that involves the less agonizing death may be "better" even though in all likelihood it isn't "for the best."