This greatly depends on how you plan on using your computer, therefore how you plan on using Xtra-PC Ultra.
If you plan to use Xtra-PC Ultra for internet browsing and streaming movies, etc, then the 16GB or 32GB would be great options.
If you plan on creating a lot of files, or copying a lot of files, photos, and/or videos, then the 64GB size is best. The Xtra-PC Ultra Pro also has added features like the FileRez software that can copy files from your old Windows HDD to the Xtra-PC Ulra Pro drive -- even if the Windows computer can no longer boot.