S5328C-PWR-SI Mainframe(24 10/100/1000Base-T,4 100/1000Base-X Combo,PoE,Chassis,Dual Slots of power,Without Flexible Card and Power Module
S5328C-PWR-EI Mainframe(24 10/100/1000Base-T,PoE,Chassis,Dual Slots of power,Without Flexible Card and Power Module)
Green is similarity.
There is your Difference. The one in RED. Based on the Link you provided. The SI has 4 100/1000 Base Combo built in within the 24 ports. From the 24 ports I assume it would have 4 that supports higher tier of connection.
It isn't a significant difference in the hardware, they are the same series and same board which a slight change - $20 difference only. Would you need the 4 100/1000 ports? If not save the $20 dollars (based on the pricing router-switch website)...
Keep in mind both are still 24 switch ports. SI just has 4 from the 24 port at higher speed. (In other words 20 + 4 for SI)