There is no difference between the two boards apart from the extra slots as you say, just depends what you use.
Not entirely true. The SX uses an ATX power connector while the LT4 uses an obsolete AT connector.
If you plan to use a modern (silent?) ATX PSU without adaptors, get the SX. There is even a tool that lets you power off the Amiga with a click from the Workbench.
