Even if both libraries were well supported in HL and by people's hardware, as the respondents are gamers it would basically come down to "who has better marketing / a better reputation?". Not a question you can draw any conclusions from, but worth quoting for propoganda purposes!
HL supports both 'proper' OpenGL (and for the era that basically means nVidia TNT class hardware) and 'MiniGL' which is basically a sub-set of OpenGL which is specifically for Voodoo class cards. Direct3D is also supported (though DX5 class drivers).
Considering that (at release) 3Dfx hardware was prolific (and remarkably good) its no suprise that OpenGL is considered as prefered. DX hardware at the time was limited to crappy S3 cards and similar Pile-O-Shite Cards (tm).
Also, given that this is users of 'Steam', ValVe's proprietry Pile-O-Shite (tm) content delivery system, this is going to be mainly hardcore gamers. So its pretty certain they'll be running nVidia or ATi cards and recent drivers.