If you can wait a few months, you'll get a host of new CPU's that will give a whole host of new options for tablets
You're not really giving any reason for an MS tablet, word and excel are simply word processors and spreadsheets. Unless you're doing something funky, I doubt your'e using any proprietary functions that are exclusive to those apps.
Ubuntu will be an option come end of year too.
At the moment iOS will give you great hardware, battery life, and a convenient form factor. The real advantage is the host of apps and hardware support
Androids main advantage is the great number of form factors, especially from companies like Asus with their transformer tabs.
MS - I'd avoid their own offering, poor battery life on the Pro and RT seems dead in the water at the moment. Looking at the new Atom processors though, and Haswell, this is where Windows Tablets will start to come into their own. Multiple choices in form factors, and desktop apps. Great for business people who are on the road and need the functionality of a desktop OS or desktop apps, while having the convenience of a tablet. The downside will be a lack of tablet centric apps, but a small trade off if you need the ability to plug the tablet into an external monitor and use a desktop OS
Ubuntu - Too early at the moment, no public offerings and the Beta is nothing more than a dev tool. End of year you'll have a mobile phone that will double up as a desktop if you plug it into an external monitor, same with their tablet offering.
If you can wait three to six months, you'll have a decent amount of choice at your disposal.
