I'd say they're both pretty even. Holmes' jab made up a more vital part of his arsenal though. Hearns had that right hand to fall back on.
Holmes wobbled and decked Ocasio with his jab. Did Hearns ever do anything like that with his jab against that caliber of opposition? The flip side, of course, is how many came close to decisioning a prime Hearns?
Hearns had a great jab, but the master of the left jab was unquestionably Larry Holmes it was the perfect balance of speed and power.
SRL dropped Wilfredo Benitez (light years better than Ocasio) solidly with a jab. Hearns didn't match this either yet outjabbed the hell out of Leonard when they fought. Such a feat means little in the overall context of great jabs. I'd say Holmes and Hearns had the two best jabs of my era. Both brilliantly effective.
Holmes' was slightly better in my opinion. He used it as his main offensive weapon, a lot of the time Hearns just used his to line up his awesome right hand, although Tommy's jab was fantastic.
IMO Hearns and Holmes' jabs were the reason they were so great on the eye as boxers. I just love a quick and solid jab. You got to love Hearns. He had a bit of Ali in him, and a bit Tyson as well.
Wow, tough question. I couldn't make an argument in favor of 1 side or the other. They are pretty much the same. P4P they would have had a great battle of the jabs.