Both are EXCEPTIONAL, but I actually slightly prefer Pacific for a few reasons. Pacific showed more on the home front,
particularly how hard it was for these men to adjust on the return home. BOB ended in Europe whereas Pacific had a whole episode devoted to their return to the states, and that episode was exceptional (the narrative thread with Sledge and his father was a highlight of the whole series). It really drove home the impact of the violence in war on the soldiers, and that is also something I felt Pacific showed better than BOB, just how much the violence of war could twist your character into something unrecognizable, and how hard it would be to prevent that or overcome that. I also thought Pacific was easier to follow with only 3 main characters that it follows throughout the series whereas BOB had episodes that focused on only one character that would never be significantly featured again, so I felt like it was more of a revolving door each episode about who you were going to see featured next. Pacific had 3 main Marines it followed, and 2 of them wrote what are widely considered the best WWII memoirs written by marines ("With the Old Breed" and "Helmet For My Pillow").
But, like I said, BOTH series are truly EXCEPTIONAL. I love them both, and they both have FAR more to praise than criticize.