The 'L' version apparently has the ability to upgrade the firmware which supposedly makes it that much better. The 'S' version I have been told is the same performance as the 'L' version without having to upgrade firmware, plus it has more RAM.
After dealing with Linksys for years... go with the ability to upgrade the firmware... They are constant putting updates out for the units and you will thank yourself later. More RAM isn't really going to help you for standard home use.