The LM3S has several technologies from 250nM to 90nM
And basiclly can work (the old one) up to 50Hz while the STM32 can work up to 72MHz
Also the LM3S has 28pin verison while the STM32 minimum pin count is 36
This why to select one of them depend a lot on the type of application - let me know more about your application and I help you to select the bst among the two.