It really does all boil down to fit, especially since you've already expressed that you want at least 105 level components.
I ride LeMonds because they fit me the best (currently I have both a
Tourmalet and Victoire). Since I race for Manchester Racing Club/Trek
St. Louis of course I'm going to suggest a LeMond or a Trek. It looks like you'd get a better deal with the Pilot vs the Tourmalet. Apparently LeMond has upgraded the frame with carbon fibre seat stays, but then downgraded from full 105 to a 105/Tiagra mix. The Pilot has full Ultegra. Liquigas
has long been using the Synapse 6 for Paris-Roubaix, so it's a bike
that can take some abuse while providing a degree of vertical
compliance. Giant's and Scott's are the two brands I have no experience with, although I can tell you that Scott is basically want Cannondale used to be (the founders of Cannondale started Scott). Giants don't usually offer many sizes, so that is something to consider.
The other factor you have to consider is that you might
want to invest in a different saddle, seatpost, and stem so that you'll
get the best fit possible, so you might want to look at a bike that comes in
below your pricepoint so you have some cash to spend on the accesories
(I've never bought a bike and ridden it, as-is, off the shelf, but I'm
also not your average american male and I'm kind of hard to fit).
Most important is whether or not the bike fits you properly. Don't know what format of the MS150 you'll be doing (short, medium, long) but you'll want a bike that you'll be comfortable on for at least 3 hours. My all Aluminum Tourmalet really starts to pound me after 3.5 hours in the saddle, where as my carbon fibre Victoire is smooth as butter mile after mile. Whatever shop you go with, make sure that they'll let you exchange the bike for a different model if you end up not being comfortable on it/not being able to get the right fit. Sometimes, even after a professional fitting in a shop, you just need to make small adjustments once you get out on the roads.
The MS150 seems to always be scheduled the same weekend as a USA Cycling race, so I haven't been able to get out and participate.