For reference, here are the known differences between the two moves. Green = better, red = worse, blue = equal, purple = different but debateable.
LEAF SHIELD:
- Base Damage: 2% (per leaf), 3.8% (throw)
- Base Angle: 361 (leaf), 45 (throw)
- Base Knockback: 15 (leaf), 50 (throw)
- Knockback Growth: 22 (leaf), 70 (throw)
- Weight-based Knockback Modifier: 0 (leaf), 0 (throw)
- Quicker start up on activation
- Quicker throw (I think quicker startup, same cooldown, based on testing in Pac Maze)
- More active frames
- Fixed, shorter range when rotating around Mega Man
- Leaf hitboxes disappear on hit
- Greater range when thrown
PLANT BARRIER:
- Base Damage: 3% (per pedal), 4.2% (throw)
- Base Angle: 45 (pedal), 45 (throw)
- Base Knockback: 20 (pedal),20 (throw)
- Knockback Growth: 35 (pedal), 35 (throw)
- Weight-based Knockback Modifier: 0 (pedal), 0 (throw)
- More start up frames on activation
- Slower throw (I think more start up frames, same cooldown, based on testing in Pac Maze)
- Fewer active frames
- Greater range when rotating around MM, undulates in and out
- Pedal hitboxes do not disappear on hit
- Less range when thrown
Here is what some of these differences mean in practice:
1. Leaf Shield has less knockback overall when rotating, which I think is better because it's easier to trap people into taking multiple hits or a grab, especially when you're trying to trap landings with it.
2. Leaf Shield has the Sakurai angle, which means it can "leaf lock" opponents who miss a ground tech. Plant Barrier cannot lock opponents.
3. With a difference of only 1% in hitbox strength, there really aren't many moves that will outprioritize Leaf Shield but not Plant Barrier, in theory. However, Plant Barrier pedal hitboxes do not disappear on hit, and they also undulate outward from MM, making them more likely to safely intercept attacks before they collide with one of MM's hurtboxes. So in practice Plant Barrier is often a better defensive option than it might seem on paper.
4. The undulation of the pedals makes them more likely to hit opponents holding onto the ledge than Leaf Shield, which often lacks the necessary range, particularly against characters whose hurtboxes remain below the lip of the stage when hanging.
5. Leaf Shield is faster, both on activation and on throw.
6. Leaf Shield has greater knockback growth than Plant Barrier for the throw attack, which has its pros and cons. On the one hand, it can make it easier to follow up on than Plant Barrier throw, especially given that Leaf Shield throw is quicker overall. Also, knocking people higher into the air is generally better for stage control. On the other hand, some people may value the lower knockback of Plant Barrier throw, because it can more easily lead into grounded followups like usmash and grab. This one is really a matter of opinion, IMO.
7. Leaf Shield has more active frames than Plant Barrier, which again has its pros and cons. On the one hand, more frames means more time to sit in shield, go for grabs, or throw. On the other hand, more frames means more time that you're locked out of your normal moveset, including UpB. It's definitely easier to self gimp with Leaf Shield, especially on stages without ledges you can wall jump off from.
