I have a amp Crate gt 15.
And i'm plugging my guitar on it. Then I turn the amp into microfone slot (pc).

I was thinking what if I plug guitar into pc (to get distort using guitar rig 3) , and then
plug the "pc" into the amp.

What is better?

Sorry for my bad english, but it's basically this:
Guitar >> amp >> pc
Guitar >> pc >> amp

>> means plugged in

What option gives me the best sound, and is there any difference?

Guitar >> amp >> pc will make the sound come out of your computer speakers

Guitar >> pc >> amp will make the sound come out of your amp

if you can, i would just plug the guitar straight into the computer without the amp. if you can't plug it straight in, then you really can't do the Guitar >> pc >> amp rig, so you're forced to do the first one
That amp has nothing desireable to offer, so just take it out of the signal completely. So:

guitar >> pc
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