I like Jace better. I'm of the philosophy that cards should be able to stand on their own, but let you do crazy things if you come at them sideways. Lab Man's fail case as a Grey Ogre just isn't anywhere close to good enough.
I like Lab Man's extra recursion potential. If I'm running a UG Lab Man deck, I want to be able to mill my deck without fear and be able to recur it with
Pulse of Murasa or a gravepulse. Additionally, this allows you to play it early if necessary to trade with some 3/2 and recover it later, or let it eat removal once in the late game. Say you have a green recursion piece in your hand but no counterspell. This allows for pseudo-protection as you can play the Lab Man now, they kill it with fire, then recur it and play it again before winning next turn, now that they're out of removal. This is a bit more feasible against decks in Gruul colors which will point any removal at your face in the form of burn. Decks with white or black removal, however, may be forced to keep creature removal stockpiled in their hand, as you are probably a control deck with few creatures, so it is unlikely to run them out.
If you're facing a fast aggro deck, having a roadblock in the form of a Grey Ogre might actually help you more than a slow 4cmc card that is sort of hard to actually cast on turn 4. Jace generates card advantage slowly while being a pinata that the opponent may or may not choose to swing at. If you're on the back foot, aggro will not waste time by attacking a Planeswalker with no effect on the board. Lab Man decks are already the epitome of a long game deck. They have some reprogrammable tendencies that allow them to grind an opponent down with CA to the point that you can kill them with anything, even a few tokens. Basically, their best-case scenario is being a control deck with an extra win-con as a safety net. They can win tight games, however, by stalling with counters, bounce, tokens, walls etc just long enough to set up an engine, mill the deck, and slap a Lab Man into play with a fail-safe or two in hand. In this case, Jace is less useful than Lab Man, because you're often frantically trying to survive and living or dying based off your ability to chain answers, as per usual with control. Jace doesn't protect himself or you, in any way. If you can afford to take a turn off in the mid-game, you are already winning. Finally, Lab Man is more splashable, has less card text, and is the OG of his kind, so more people are familiar with him, reducing players' comprehension time. Don't support the Jacetice League; keep our mad scientist buddy in business!