Get cross bars if you even think that an RTT is in your future. I had to get cross bars added to mine. Yes you lose a little bit of space, but there is still plenty for firewood, more bins/boxes, etc. i've never used the entire length of the rack at once (except for my RTT) so do you really think you need THAT much real estate for stuff on your roof? you'll likely fill up the cargo area of the rig first, before loading up your roof.
Don't get the removeable sunroof insert. I've never taken it out so it's been a waste of money. If you don't use your sunroof now, you still won't with a rack on. Plus, it's a nice security feature. You'll get to leave your sunroof wide open while you are doing your shopping or whatever so your truck doesn't become an oven.