Apologies, I meant to point out that both games have absolutely horrible experience of exploration, hence "exploratory". So I think to name one dead awful experience as an upper hand over another dead awful experience is just a lie. IMO
Fallout 3 had more graphically varied exploration, but the things to do are always the same. Mass Effect had superficially varied graphics and the things to do were always the same. Since I care more for other elements than graphics, the experience of exploration is just as bad in both games.
Well, then we just disagree
I care about graphics a lot, but less so the actual quality of graphics. I'm talking about variety - which is just as big a part of exploration as anything else.
Fallout 3 had a lot of unique areas and I mean BEYOND simply graphical variety. I'm talking about story and characters. You'd stumble upon audio tapes with unique voice acting that told you to go somewhere and find something, you'd often find computers with relatively "meaty" log entries that detail something story-related, and you'd just as often find NPCs that had no relation to the main quest. I found several "puzzles" in the shape of said log entires or some trigger that activated something else.
I don't know exactly what you're looking for, but to me Fallout 3 represents some of the very best exploration I've seen in recent years. Considering the scope of the game, I came away very impressed by this aspect of it.
I'm interested to know what other games you've played that have superior "exploratory" elements. I can't think of many, especially not open world non-linear games. Even the Gothic franchise doesn't hold up in terms of the sheer amount of unique locations with non-quest related lore.
Mass Effect, on the other hand, had very weak exploration. All the side-quests were basically rehashes of the same 2-3 texture sets on a handful of planets. Story elements were slim and mostly text-based, and since all planets were empty except for the 2-3 locations meant to be explored - it all added up to a bland experience. The main quest areas were well done, but all were overly linear and predictable. But that's par for the course, and even KOTOR (my personal favorite Bioware CRPG) suffered from the same linear structure. The only non-linearity is that you can choose which planet/area to explore first, basically.