Looking through my faves list, it seems I prefer fake boobs. 39 silicone girls to 25 naturals. Though personally, I think I just love big boobs period, whether they're real or not. (Doing some number crunching, it seems that on average, I like girls with 36E mesurements. Though I've been known to go as high as L.)
Personally I prefer the gel-based boobs, much softer to the touch with most natural appearance than silicon boobs, anyway I find that some pornstar have ridiculous breast implants, disproportionate to their body size.