I generally prefer the Oxford World's Classics series to Penguin's editions of the same works and authors. Why? Because Oxford boasts better (thicker) paper, better fonts, better printing, better covers, usually better notes and better introductions... Oxford just seems to present an overall better product at roughly equivalent prices. But Oxford made a crucial mistake in their edition of Lord Byron: The Major Works--they didn't give Don Juan a separate volume. The effect of stuffing Don Juan into this volume means that the book is conflated to an unwieldy 1100+ pages, and several of Byron's key poems are either omitted or severely abridged (like Lara and The Corsair). Here, they really should've followed Penguin's lead in creating separate volumes for Don Juan and another for Byron's other poetry. But it's even worse when one considers that the Oxford contains a sampling of Byron's prose. So after you subtract the pages for Don Juan and the prose, you're left with only about 470 pages of Byron's other poetry in compared to Penguin's 780.
Now, granted, perhaps what's available here in the Oxford Edition will be enough for many readers, and it does still provide its usual advantages in paper, printing, font, notes, and intros. Byron was incredibly prolific, but like most prolific poets he tended to produce more bad poetry than good/great poetry. It's just a numbers thing; writing great poetry takes time and attention to small details. It's why it took Milton years to write Paradise Lost at a rate of 40-or-so lines a day. Every detail had to be worked out. At Byron's best he was as good as anybody, and his skill combined with his unique philosophical worldview makes him endlessly provocative, compelling, and readable, even at his worst. Byron didn't believe in Pope's maxim about how the real art of poetry was in rewriting and perfecting what one had written. He rarely tried to better his drafts, preferring to move on to the next project. I think this approach works best in his longer works where minor imperfections in the verse--be they occasionally bland, prose-like formulations, awkward meter, et al.--were less noticeable when set against the macro vision of the narrative and characters.
But, in light of realizing that Byron was at his best in the longer pieces, it's precisely those that are hurt most in The Oxford Edition. Lara and The Corsair are essential Byron, even if they're not as great as Don Juan or Child Harold's Pilgrimage, and they're almost non-existent here. And lesser (but still quality) works like The Siege of Corinth and The Prisoner of Chillon are gone entirely. So, I'll leave it up to each individual customer to decide if the Oxford's usual strengths compensate for the loss of these works. Another option is the Norton Critical Edition, which is more valuable for its critical apparatus than for the poetry itself.