If you're looking for a backup pistol to carry around, I'd say it would have to be the TPX. The SA is just much too big. Also the TPX doesn't puncture the CO2 until first trigger pull, so you know it won't be empty due to a leak when you need to use it. Yes it's more expensive but it's smaller/lighter making it nicer to carry around all day and the mags make it quicker/easier to reload.
If you're considering wanting to use the marker as a primary, then the SA is definitely more flexible to use (stock, mock suppressor and RipClip turns it into a nice rifle).