I believe in general GDMs aren't overly aggressive, but ultimately it'd be pretty irrelevant anyway because they're super small (like their actual thickness). Everything I've read has said they're not even a potential threat to anything except small gobies, dartfish and blennies. SFE can actually get large and thick and are capable of consuming or just simply killing much larger prey (either intentionally or during feeding).
The only downside to GDMs is their cost. Which is usually close to 10x that of a SFE depending on your source. As for feeding any eel, I'm not sure I've heard of them eating pellets ever. I believe that it's only either frozen or live.
Totally my personal opinion, but get a GDM if you can swing it. I'm biased though because I desperately want one. Unfortunately, I don't have a suitable tank for one (it'd either have to be with frogfish or gobies/firefish) so I'm stuck living vicariously through other people for now.