What's the better way to expand my small SSD?
A traditional USB flash stick will not cut it for your purposes. This is not a question of R/W speed. Regardless of the R/W speeds (I own an excellent 128GB PNY Turbo stick), the on-board controller (the microchip that serves as the brains behind the drive’s operation) is built for low-cycle I/O operations. If you plan to run virtualization software from an external drive, with portability/size being the caveat, your best bet is purchasing an mSATA-based SSD with an exclosure. 

Here’s a good image to put the size into perspective:



Bottom line: you won’t be able to reliably run a VM from a USB based flash drive. You need a dedicated external drive with a controller engineered to handle the operations you plan to execute.