I am running Windows XP Pro under Parallels Desktop on my mid-2011 MBA, and also the 32-bit version of Windows 7 Ultimate via Bootcamp.
I set aside 25GB for Win7 Ultimate with applications and at the moment this is about half used (any unused capacity is effectively lost to me as I can't use it for OS X or anything else). Bootcamp switching between OS X and Win7 isn't at all immediate, you need to close down OS X and load Windows, then load the app, do the work, close down Windows and reload OS X. No fun if it's more than once a day.
My Parallels virtual machine with Win XP is currently just under 8GB with applications (the size of the VM is dynamic and can expand and contract as needed, so no wasted storage space). Thanks to the speed of the SSD, loading Win XP apps under Parallels is faster on my MBA than it ever was on my desktop Windows boxes!
If Parallels isn't already running, loading Photoshop (including loading Win XP first) takes 17 seconds. If Parallels is running in the background it takes about 7 seconds if it's the first load since the VM was started, otherwise it's under 3 seconds.
Best of all, you can carry on using other Mac applications whilst running the XP apps.