Personally, my decision is mostly character dependent and somewhat reliant on stage position. Neither are definitively better than the other, but I found some good traits in both. It comes down to preference for the most part
Base notes
Grounded gives longer WD but takes 4 frames longer because jump squat. Easier to react to on hit / shield, so starting up shield pressure is better with grounded IMO
Aerial allows for almost instant aerials that usually connect on opponents at any % if it hits. On shield, double mags / mag -> retreating DJC aerial, etc. are much easier to do than leading in with grounded mag.
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Adding to what's been said....
Grounded: Opponents that like to attack after being hit. Characters include Luigi, Yoshi, Bowser, Peach, etc. Grounded mag leads to Utilt (since they won't keep DI'ing away). Time & space the disjoint to not trade with attacks and so you get the sweetspot.
Lower %'s so you can SH -> aerial. Bair on fatties / fast fallers under 30, then Uair roughly 30-80, then Bair or something. Haven't fully labbed that yet.
Lower %'s against lighter weights gets you those Utilts that convert better than Uair because of extra hitstun (Marth can counter out, Yoshi can DJC out under 40% or so if you're not perfect, etc)
Higher %'s / other: I find grounded mag -> WD back allows easier reads on the enemy's DI. Allows ample time to decide on DACUS, Fair, RAR Bair, running Uair, etc.
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Aerial mag: Used more on heavies / FFers. Mag -> Bair is much easier on heavier / larger characters. Catching opponents hard DI'ing in towards the stage around 70-100% can get caught by Mag -> rising Uair -> Bair, but less get followed up by DJC Uair after 70% due to knockback & positioning. Plus, I've found less followups after ~70% by aerial Mag -> DJC Uair, whereas DJC Uair is better at lower %'s
Best use of aerial mag is either front shield pressure or getting instant conversions to an aerial attack if you know how your opponent will react