SFU is solar flux unit(s). SFI is the solar flux index.
A SFU is a measurement standard, The SFI is the level the solar flux is at and being reported.
Basically they are used the same way. IF the SFI is 150, then it is reporting a level of 150 Solar flux units.
Higher is better.
A index and K index you want to see as low.
We want sunspots (cooled spots on the sun) but no flares as they raise the A and K levels.
Unfortunately sunspots can give way to solar flares.
Here is a good simple explanation;http://www.k7pt.com/solarphp.php