Minor Spoilers !
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Hi there !
The answer is ... neither !
Well, of course, it depends on your party's make-up and your playing style as the wise Melusine and others have pointed out before me.
In my party (fighter/mage pc, Minsc, Aerie, Jaheira, Imoen/Jan, Keldorn/or various) Minsc obviously fills the role of the main tank. Especially at higher levels, when everybody else has got lots of cool spells to cast, he is often the only one upfront, taking all the heat in melee. With a constellation like that it is most important that Minsc can stay there as long as possible, so that the others won't be disrupted while spellcasting or, almost worse, have to disrupt it themselves, because they have to bail out Minsc when he gets into trouble. So, with him the emphasis shifts from offense to defence.
Now take into account that the shields in this game are just awesome. We are talking about a difference of at least 5 ac points here, and that translates into a lot of hits that could be avoided. On top of that, on the defensive end (dex ac bonus, hps) Minsc is rather average by the lofty standards set by the NPCs in this game. IMNSHO he is therefore not ideally suited for either two-handed or dual-wielding fighting, even considering that as a ranger he already comes with two proficiency slots in dual-wielding. I think shieldless fighting is best left to fighter/mages as with them, because of their protective spells like stoneskins, mirror image, etc. the ac does not matter that much, or to a character who is only a second or third option in melee-fighting. So disregard all of the above, if in your party Minsc happens to be the fifth best fighter only.
I also think that dual-wielding is very overrated, probably because of its "macho" appeal. It has two major advantages: the extra attack per round and that two wielded/equipped weapons can give the benefits of two special abilities/boni of magical weapons. Especially at higher levels, I don't think that the advantage of 3.5 attacks over 2.5 attacks per round outweighs the drawbacks compared to fighting with a shield (some have nice extra equipped abilities, too, btw) or two-handed fighting. Two-handed fighting allows you to have equipped one of the great bows or crossbows (equipped with the bracelets of dexterity Keldorn is a killer with the crossbow) and one of the even better two-handed swords at the same time, which grants flexibility. Two proficiency points put into two-handed fighting give you a doubled chance for a critical hit, an extra point of damage and a plus 4 weapon speed bonus. Against tough opposition that is probably hard to hit, IMO the doubled chance of a critical hit is much more important than the extra attack, because an attack can miss, but once you hit with the two-handed weapon you are now double as likely to do doubled damage. Therefore, against tough opposition, ergo when it matters most, you might well have a faster kill with a two-handed sword, which are also the weapons that inflict the most damage per hit, than with two weapons wielded simultaneously.
On top of that: in the case of Minsc, incidentally, one of the best two-handed swords in the game also covers very nicely for one of his major liabilities, if you know what I mean.
I, finally, rest my case.
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So long !
R³ - Co-president(s) of the Club of Broken Hearts