Stick Pro Duo
The Memory Stick PRO would be the longer-lasting solution to the space problem. Most devices that use the original Memory Sticks support both the original and PRO sticks since both formats have identical form factors.
Memory Stick PROs have a marginally higher transfer speed and a maximum theoretical capacity of 32 GB, although it appears capacities higher than 4GB are only available in Duo and PRO Duo form factors. High Speed Memory Stick PROs are available, and newer devices support this High Speed mode, allowing for faster file transfers.
All Memory Stick PROs larger than 1 GB support this High Speed mode, and High Speed Memory Stick Pros are backwards-compatible with devices that don't support the High Speed mode. High capacity memory sticks such as the 4 GB versions are expensive compared to other types of flash memory such as SD cards and CompactFlash.