Short answer:  All of the above. 

Long winded answer:  Depends on what I'm writing.  In settings of older times, I sometimes lean to magic.  More current ones, I use science (which seems to have the lioness' share of transformations in my stories, I admit), and in one story I have the two different elements duke it out for dominance.  And there's one where we have to guess, I suppose.

Just one thing:  I'd be reluctant to call something "reality" based just because it has science-nature based mechanics behind it when it's ridiculously fast in happening or huge in effects.  I mean... from 5'2" to 11'8" in less than 60 seconds?  Tiny frail waifs to hulking Amazons? From A-cups to weather balloons before the commercial's over? From one sip from a test tube?  That's "realistic?" 

I'm with Jimmy on this one.  All of the above.

For me it depends on the story and the concept and what I'm trying to do with the finished product.  Sometimes I do leave some of it to the readers imagination, so that they can fill in gaps that would be more exciting to them, and different from my ideas, but mostly I direct the readers where I want them to go.

I took a look at my stories and it seems I lean towards the Science/Reality category (62%).  Again, it just depends on the story and some stories blur the line. 
- My "Monkey Paw" story is magic but the wishes are based in science.  Go figure...

I enjoy both, but to be honest the science in my stories is really just another form of magic that pretends to be science.  The difference is in how the user understands it, and to what extent the power that the science or magic brings is universally available.

At bottom, the key in my stories is power, and science or magic are different ways to exercise power, with different rules.