I'd go with the W7 equipped netbook. It has better hardware and you'll find more compatibility with various programs that you may want to install as time goes forward. Plus, the starter edition shouldn't tax it too much in terms of performance.
I think the choice should be clear both in terms of hardware and software.
The Sony is better equipped hardware-wise with a good amount of ram and a good (netbook) processor. Personally I am a fan of Solid State drives in netbooks but I digress.
For the OS, Windows 7 Starter is an excellent, efficient, and current Operating System that works very nicely on netbooks. If you feel so inclined to unlock some of the Home Premium features one can upgrade by merely inputting a code (~$80) and waiting on average 15 minutes and Voila! Compatibility as buwish points out will be far better for most programs and of course, Windows 7 is supported OS versus XP which is soon to be not supported by Microsoft.
In conclusion, go for the Sony M series and don't look back!
In my opinion, microsoft is already pulling away from XP. My wife's netbook with Xp is continually having problems, regardless of how many updates we do. It is a fine machine, but the OS is falling behind the times and she is starting to hate XP. I am too, as I spend a lot of time working on older machines for folks.
The fact is WIN7 has been hugely successful for MS, so they will not feel compelled to continue supporting xp. That was not the case with vista, but the landscape has changed.