IMO there is not one specifically that is better, maybe easier to learn or say better for someone who has little experience. WIth that said I might say the deads variation are perhaps safer. Some guys perhaps more squat dominant may find the GM better and a guy built for pulling may find an rdl or sldl more to his liking. It does not mean you cannot alternate or learn either, but I might find the GM is a bit more advanced and harder to learn to do effectively for some and it seems larger waisted guys have more advantage too perhaps.
Personally I find the RDL quite good in working the hams and glutes as you do not rest the bar on the floor after each rep but keep constant tension on the muscles. That can be different in the sldl, which also utilizes more low back and bending some from the waist, also depending on how you perform them. Some feel too, there is a bit more control with a deadlift, having the bar in the hands.
The GM with weight on shoulders can IMO be a bit more advanced if done right, since you are not holding onto the bar and cannot just drop it if you run into trouble. Also should always be done in a rack with spotters and or pins set for a catch. The GM also puts more pressure on the abs so as to keep stable when dipping forward, raises blood pressure in the head more and makes the lever arm from hips to weight longer as you bend over, so more static strength or arch (if you are doing arch back GM's) has to be considered.
There can be a few differing ways to do the GM also, by sliding the butt back and stretching out the hams & glutes, then tensing/shortening them to upright the torso. I have also done a more bend at the waist type where the low back gets more involvement. Again there a re a few ways to do GM's and deads to elicit cdertain parts of the posterior chain.
A deadlift also involves scapular retraction of the shoulders so the weight moves or swings ever so slightly being held in the hands, which does not come into play in the GM. The weight stays in one place on the shoulders.
Personally, if I was new at either of these, I would do the deads (which is really how I started and I am more pull oriented) until I built some lower back, ab, ham & glute strength and then if one feels the GM needs to be done or tried, add it in "slowly and or with light weights" getting used to the ROM and cementing the form down before doing it. Don't do it because you see pics/vids of big PL'ers working it and it looks cool or is a fad blowing thru your gym.
Lastly and to add, if you are not real experienced with knowing the difference between bending at the waist and bending at the hips, it may be a good thing to take a little time and feel the differences without any weights at all.
You can see how stretching the hams and glutes by moving the butt back while keeping the low back flat and static is different than moving from the waist and using the low back to aid in the uprighting of the torso.