I´ve seen some really amazing footage from the 17,5mm f0.95 but I keep wondering if it´s really worth it over the Panasonic 25mm 1.4 or the 20mm 1.7. What do you guys think? The 17,5mm costs around us$1300, which is double the 25mm and more than double the 20mm.
Realistically the F0.95 is only going to have use in very limited situations and if your reasoning is to shoot extreme low light, or for some artistic choice then it might hold value to you.
The issue in your question that I see is you're asking about lenses in relation to F/Stop though you're comparing a 17mm, a 20mm and a 25mm lens. All of these will look different and whilst I appreciate "these lenses" all seem to sit in a particular category of comparison - what you choose will make a big difference - well, big might be too strong, relatively that'll depend on your perception.
I'd however be asking myself which of those lens lengths I actually want to use the most, being as the choice is to choose just one for now. Secondary to that would be lens speed.
FOV wise my favorite is 35mm. Speed wise I´m trying to compensate the fact that I get clean images at iso 1600 with the NEX-5N when shooting stills (I have even some very usable iso 6400). I know I won´t be able to push the envelope so much with the GH2.