Both have their own good and bad side

One cannot opt Between Ms-office/Google docs only on cost basis..
Some more factors which Gains attention in this context is that

None can assure you of !00% Security over cloud storage( as it is still in its nascent stage),
Added to that
Privacy always matters (People tend to keep data private than to float on cloud)
Cloud storage is an advantage to enterprise level and not to an individual .

One advantage i found that Google has over Microsoft is that , they can provide instant updates.

So, please do research about which suits better to you then go for it