Both updates provide security enhancements among others
It looks like, beginning January 31, T-Mobile customers who own the Galaxy S II and/or Galaxy S III will receive software updates.These are minor updates that provide security enhancements along with some tweaks and improvements. Both are pushed OTA (over-the -air) in waves, so not all users might receive them at the same time.
According to T-Mobile, Samsung Galaxy S II will receive the following improvements and fixes via the new update: Qualcomm fix, security enhancements and Vlingo S Voice improvements. T-Mobile Galaxy S III gets the following changes: security enhancements and improved user experience.
Keep in mind that you need to be on a certain version of Android in order to be eligible for these updates, so make sure you check the official announcements for Galaxy S II and Galaxy S III.