This whole question doesn't even make sense. Why?
Because all science fiction is fantasy, but not all fantasy is science fiction.
(Okay, stay with me on this one)
Fantasy is a broad genre which covers "things that are not possible in today's world or through verifiable history", i.e "fantasy" as opposed to "reality".
Science fiction is a fantasy sub-genre - fantasy concerning the future or "futuristic" elements, technically might not actually be the future per se but the common theme is technology that does things today's technology cannot.
Medieval fantasy is also a fantasy sub-genre, the common theme being a certain adherence to medieval aesthetics, clothes, weaponry, creatures from medieval folk tales such as dragons etc. May or may not actually be in the past, and sometimes medieval and science fiction elements of fantasy merge (like in for instance, Anne McCaffrey's books).
Steampunk is also a fantasy sub-genre, it borrows a little from both science fiction and medieval fantasy, but it also has a style of its own.
If your question is, instead - and I think this is what you really meant to ask - "is steampunk more medieval or more on the sci-fi side?"... well, that depends on who's telling the story and where and how they choose to tell their steampunk tale. Bioshock for my money leans toward the sci-fi side. Thief leans for me more towards the medieval side. It all depends, there is no definitive answer to this question, you have to take it on a case by case basis as the lines blur between these sub-genres often.
Because all science fiction is fantasy, but not all fantasy is science fiction.
(Okay, stay with me on this one)
Fantasy is a broad genre which covers "things that are not possible in today's world or through verifiable history", i.e "fantasy" as opposed to "reality".
Science fiction is a fantasy sub-genre - fantasy concerning the future or "futuristic" elements, technically might not actually be the future per se but the common theme is technology that does things today's technology cannot.
Medieval fantasy is also a fantasy sub-genre, the common theme being a certain adherence to medieval aesthetics, clothes, weaponry, creatures from medieval folk tales such as dragons etc. May or may not actually be in the past, and sometimes medieval and science fiction elements of fantasy merge (like in for instance, Anne McCaffrey's books).
Steampunk is also a fantasy sub-genre, it borrows a little from both science fiction and medieval fantasy, but it also has a style of its own.
If your question is, instead - and I think this is what you really meant to ask - "is steampunk more medieval or more on the sci-fi side?"... well, that depends on who's telling the story and where and how they choose to tell their steampunk tale. Bioshock for my money leans toward the sci-fi side. Thief leans for me more towards the medieval side. It all depends, there is no definitive answer to this question, you have to take it on a case by case basis as the lines blur between these sub-genres often.