the software itself is almost identical...you do everything in premiere that you can in FCP.
the difference is in the convenience and ease in which you can do it. most PCs are too slow to handle the video so you notice a lag when editing and playing the video. Premiere freezes a lot more often because windows can't deal with all the information, forcing you to quit the program without saving. when I was editing on a PC I could only have Premiere running or the whole computer would crash....now that I have a Mac I can edit in FCP, listen to music, check my emails, download porn, go on skype, etc all at the same time with no problems.
the end result of your video will be the same whether you use FCP or premiere...you'll just get it done faster and easier in FCP. if you're trying to save money and already have a PC, you're best to upgrade your graphics card and RAM instead of switching to Mac. if you're planning on buying a new computer to edit on and you're debating between a mac and a PC, spend a bit extra and get the Mac, it will be less hassles in the long run.