In pleasure they will be looking more at the horse than they will you. Does he move along nicely and willingly? Does he have smooth transitions between the different gaits? Etc. You can typically have a little longer rein to really let them stretch their head down a bit. Not too far but the judges usually don't like a horse that has his head high up in the air.
In eq., they are watching the rider more than the horse. Your reins should be shorter and you should be sitting up a bit more than you would in pleasure. Be sure to have great heels and your toes pointing towards the horses nose. Quiet hands, nice position, etc. Also, the judges might ask you to do other things that they wouldn't in pleasure, like sitting trot. I have even heard of judges making you dismount and remount, or dismount and switch horses but that's not likely. If you can, watch an eq. class in another division to see what the judges are asking of the riders so you know what to prepare yourself for when you go in.