Is SI or SE better for Corsairs?

None: 2412.7% =3.8*4*1000/630
SI: 2533.33% =3.8*4*1000/600
SE: 2546.03% =4.01*4*1000/630
Both: 2673.33% =4.01*4*1000/600

Agreed. Technically SE is slightly better, but from a training perspective, SI might be a little better.

If an extra crit or two does not help you kill faster (i.e. 1 hit instead of 2 hit), then SE is near pointless. Also, SI will cause you to attack slightly faster so you will kill all monsters slightly faster, not having to rely on crits for that. I suppose it might matter on your particular case on which is better (if SE helps you 1hit/2hit more often, then it is probably better) but having either or can do you no wrong.