Well... it depends.
At 1k, if you use shaders then D3d will win for one really good reason, you don't need to import functions by hand like you do with OGL under windows. Also D3d offers asm for shaders.
No shaders ? Its really about 50/50. In fact you could use them together. I think I got them both running together in about 600 bytes once.
At 4k, d3d has a very slight edge but to be honest its lost in the noise compared to good coding. Also you can use all of DX with OGL (eg sound, maths etc) and its quite easy. I'd say d3dx is cool though and offers a lot OGL does not have (raytracing, subdivision surfaces etc).
Chris