Microsoft has detailed the storage of Microsoft Surface with Windows RT. Surprisingly (for me), if you owned a 32GB Microsoft Surface, you will only get about 16GB left for storage and other things. A whopping 16GB had been used for the OS and other programs.
Based on the table provided by Microsoft, you can see that the OS, MS Office and other apps took out 8 GB. In exchange, you get a really nice Microsoft Office experience. And there is a 5GB used up for recovery. Both recovery has the same storage size for 32GB and 64GB.
So, it will be logical to get the 64GB Microsoft Surface if you intend to store many things or download many apps.