obi knoobie here. I know that there are pros out there that can help you better.
have you tried both? importing a sample HD footage and try editing/rendering it in FCP and Vegas?
actually i vouch for both. I like the vegas, evolutionary development from the era of cool edit pro. Just like it if i were to edit on the MS platform. I used premiere, its good yes!! no doubt about it. just a matter of ease of use for ME..im not into flaming anyone using Adobe P.
As for FCP, the de facto editing system. its apple anyway.
need fast? how about stability? ones that dont crash as much. it all depends on HD speeds, memory also. Same like editing photos now..between using Aperture and Lightroom..most of the time, file size and your system specs matters.
in the end of the day, the ease of use, reliability and of course speed matters...to you. if you ask me, the completeness and reliability of FCS enticed me to use it now in my MBP.
maybe i am biased but if i were to use a PC, then i go for (to) vegas.
your call..try them urself using similar HW specs