I would think that 2007 would be better because Microsoft will support it much longer. I had to install XP on my wife's 2006 MacBook Pro since Apple said that hardware would not support Windows 2007. I found that Microsoft stopped shipping XP in June of 2009 and found a few copies of OEM version after searching awhile. Microsoft also said they are no longer supporting XP unless is the SP3 version. If I had a choice I would go for 2007 since that version is readily available and supported.
Apple is coming out with a new operating system in 2011 and whenever that happens, older software can have problems.
So far, all my important programs are happy running on my MacBook Pro. (Well they should be, that's why I bought it!). However, like most people who have migrated I have things like PC Games I would like to keep playing, and am considering installing Windows either via Bootcamp or Fusion. More likely the former for the DirectX compatibility.
Therefore, is it better to go with the older, but lighter, XP or the latest version of Win 7? OSX will definitely remain the main os, as I said this is more for gaming on the side.