I ride a newer XP and prefer standing over sitting. I feel more comfortable and depending on the conditions, I feel safer too. I will sit when we get into the tight twisties for a lower center of gravity, but that's about it. There are advantages and disadvantages to both, but as many have stated, it's not about looking cool as much as it is personal preference. Doo what you doo and don't worry about what others think.
As for the backpack question, I got a Klim nac pac for Christmas and love it. My sled has NO storage since I removed the trunk that was attached to the back of my seat. I don't carry a gourmet meal, but an extra pair of gloves, spark plugs, quart of oil, and a few tools is about it for me. It also comes in handy when I need take a layer of clothes off when I get hot.