Well, Samsung's Power Saving Mode consists of 4 features:
CPU Power Saving: Limits the maximum performance of the processor (may also disable some cores).
Screen Power Saving: Uses a low power level for the screen.
Background Color: Uses a dark background color for some apps, which might reduce power consumption on some screens (see here).
Turn off haptic feedback: It's the vibrating if you use the hardware keys or type something on the virtual keyboard.
JuiceDefender consists of several other features which might save some battery, if correctly configured:
It lets you control when to use WLAN/2G/3G connection, so you don't need to change them manually (It can be configured to disable all 3G and WLAN at night for example).
It has location aware WLAN control, so it can automatically enable/disable WLAN when it's available in this location.
It can also do CPU-Scaling, limiting your CPU power when your phone is in idle state.
* Some features might not work because they need a specific ROM.
So JuiceDefender can save a lot of battery life if properly configured and will only slow down your phone when it's not in use (only if configured properly). (CPU) Power Saving will definitely reduce your performance.