What is the difference between EW-SD300 and EW-SD300-I wires?

There isn't one, really. The EW-SD300-I version of the 12-speed Di2 wire comes with wire holders or Di2 tie-wraps, and that's it.

Internally, both versions are exactly the same, and both versions will work just fine on your 12-speed Di2 bike. Personally, I tend to buy whichever is cheapest - I have plenty of Di2 wire holders lying around myself.