It depends.
I'll assume here that we're looking at planet earth identical in almost every way but with weaker gravity.
Basically, let's begin with the differences: what is up with each arrangement?
Basically, plantigrade legs offer much more stability and higher surface area, this can still allow a creature to be efficient at walking and have much better balance. However, plantigrade legs come short when it comes to running at higher speeds. It's generally better if you need to move around in 3 dimensions, if you need to have sure footing or if you're using your legs to swim. They can also allow for a more flexible leg.
Digitigrade are the opposite basically. They're great for running faster and with good efficiency, but offer less surface area and are generally less stable when you need to balance yourself. They're great for running fast in 2 dimensions and can allow for very strong kicks,but being structured for running means that, by default, they'll be less flexible.
So, plantigrade or digitigeade predator? It's not a straightforward answer. Depending on how and where they hunt, things can change quite a bit. Let's go over 2 examples:
1- Your creature can fly (on such a scenario, lower gravity means you can be bulkier than you normally could, as the pull of gravity being weaker makes taking it to the skies much easier). On such a scenario, how you'll take down your prey will also tell how your legs are. If it has the body plan of a bird, then it's probably going to be digitigrade with eagle or owl-like legs. Of however it uses its Jaws like a spectral bat, it's legs might be kept plantigrade since they aren't as needed. The type of wing might also interfere with that, since as far as I know all bats and the majority of larger, more human-sized pterosaurs were plantigrade (which would make in your bipedal creature, since it'll need more stability without the forelimbs to help walking) while all birds descend from dogitigrade ancestors and thus kept such a limb arrangement.
2-your creature lives and Hunts in dense forests and can't fly. Plantigrade. In such an environment running is pointless, much like a flying creature, when you move, you do so in 3 dimensions, and with less gravity to pull you down you can still hold onto weaker branches without falling. In such a scenario you don't want to be good at running on the ground, you want to be nimble and good at grabbing, things you can do much better with plantigrade hind limbs (which you can litterally turn into a second pair of dexterous hands to move around).
3- your creature lives on plains and runs after prey. By default I'd say digitigrade, but unlike the other 2 cases, in here the lower gravity is a problem. You'll take longer to reach the ground, and while that can mean some long running gates, it also means there's a good window of time between each time you get to push the ground, meaning running creatures will probably need better ways to get as much traction as possible. Claws that dig into the ground like those of an ostrich come to mind.
I'd still say that digitigrade would be overall better than plantigrade, but this should illustrate my point well: lower gravity negatively affects you specializations for running while buffing adaptations to fly and climb. If your Martian needs to run after prey, it'll probably still be digitirade (like an ostrich, emu or cassowary, not like a cat or a dog. Mind how the feet of the latter ones are adapted to a quadruped).
Overall it depends a lot on what your creature Hunts, how and where. You want to awkwardly pursue prey on the ground? Digitigrade might be best. You want to climb at them at terrifyingly efficient speeds below the forest canopy? Platigrade. You're an aerial predator that hits what's more stuck to the ground than you? It might vary on what you use to kill them (your legs, with feathery wings? Probably digitigrade like a bird's. You use your Jaws and membranous wings? Plantograde might be a better fit).
