The toss up between 70-620 and 70-270 comes down to your preference between XP and Vista. If your company is regularly using XP, of which quite a few companies still are, then you should seriously consider taking that.
The 70-270 exam focuses on Installing, Configuring, and Administering Microsoft Windows XP Professional. Whilst the 70-620 exam focuses on Configuring Microsoft Windows Vista Client. Both of these are similar skill sets just applied to a different technology platform. The 620 is very easy in comparison, but if you have no use for it, then you shouldn't take it.
Microsoft have also just introduced the new Windows 7 Exams, so it would probably be worth taking the XP exam in the meantime because your skill sets already match those, and then moving to Windows 7 at a later stage. Remember, its not worth relearning a new technology if you already know another one. Even though that technology might be slightly older it is better to get a certification under your belt than wait to lean a whole new technology.