Infraction: 6. Do not post unless you have something new and worthwhile to say.
Message to User:
Just report wrong information instead of adding to the mess.
You still want instant on the MH because you have a higher chance to hit with your MH, and unless you're hard-capped with hit rating (you shouldn't be) then that makes a difference.
No, you want deadly mainhand as it has way higher proc rate with imp poison & envenom than IP with a 1.4 speed. Although the only thing affected by it would be envenom so the overall damage difference isn't going to be very noticeable.
Your mainhand and offhand hit chance is the same.