Well, they're pretty similar phones.
The main differences are:
-SP has a bigger battery (~600mAh)
-SL has the "better" camera (12MP vs 8MP)
-SL has more pixels per inch because of the slightly smaller screen (4.3in&342ppi vs 4.6in&319ppi)
-SL has 32GB of internal storage but no removable SD card slot. SP has 8GB of internal memory, but supports up to 32GB sd cards.
I would choose the SP and get a 32GB memory card.
If you can't get a memory card, then the SL.