Welcome to Epic. Having done P to V a couple of times (on a pair of twin tip 1080s the first time since i didnt have a race ski with me! :) ), you can't go too far wrong with any of these and they will all be a significant improvement over your Volkls.
You also need to consider what length/radius you are thinking about. All of these come in a 23m @ 183 womens wc version as well as the 27m in longer lengths. (the Atomic and Fischer can be had in both radii @ 183). At your height/weight you may want to look at the 186/188 27m versions. Unless you have spent a lot of time training and on a full on race ski I would suggest staying away from the 190/191 versions - 5-6 minutes in a course on one of these will give you a decent work out!. I have skied (and own) both the Fischer (in 183 23 and 27m (much prefer the 27m) and 188) and the Atomic in 183/184. Haven't skied the head. They are both great skis, but with a bit different feel and i don't think you would be disappointed with either. The Head is a bit stiffer i believe and I would probably stay away from it.
But remember, these are modern GS race skis - they are NOT your 70s GS skis and require a whole new learning curve to be quick on them. To get them to work properly you need to load them up right, drive them and get the right body angles, otherwise they will feel sluggish and reluctant to turn in and be slower. You also need to exercise a bit of care when you relax on them as you invariably need to do at times on P to V - nobody can keep consistently carving all the way ! - so watch they don't bite you!
if you have the opportunity try them both but this is usually pretty difficult. If not, pick one and just get used to it. Your time will not be markedly different on either one. BTW, if you are looking for a good deal, I have a 2010 184 Atomic D2 race stock and a 2011 183 Fischer WC GS available, both have been pretty lightly used and in great shape . Lots of people here that have bought from me in the past and can confirm it is good gear!
Oh, and one last thing, I make the assumption you are using a race suit, hugely important on that length of course!
Edited by ScotsSkier - 8/24/12 at 2:21pm