I bought a 128GB stick to back up my MacBook Air (Time Machine). This way I don't need to re-format my external HDD I use for backing up files from my Windows machine. Since I won't be using the stick with my Windows machine I thought I might format it with a "better" file system than exFat. Would you recommend formatting it to HFS+?
From Apple KB : Backup disks you can use with Time Machine
Formatting a backup disk for use with Time Machine
The most common format for a Time Machine backup disk is Mac OS Extended (Journaled) format.
Time Machine also supports Mac OS Extended (Case-sensitive, Journaled), Mac OS Extended (Case-sensitive, Journaled, Encrypted), and Xsan formats.
If your disk uses the Master Boot Record (MBR) partition type, some partitions might not be available for use with Time Machine.
In short, use a Journaled HFS+ partition on a GUID[GPT] formatted disk for best results.