Its been several years since I rented a penske but it was not a good experience. The truck was falling apart. Certain critical guages didn't work and I got a D.I.L.L.I.G.A.F. response from the people behind the counter.
Penske uses or used to use speed limiters set to 65 MPH. That may seem reasonable but there is always going to be someone who wants to go even slower than that and there is not enough speed to be able to pass someone without causing a traffic jam in the process.
About gas milage, you won't seen much, if any difference. These are not small volkswagen turbo diesels. Also diesel costs more. If you're more comfortable in the smaller truck use that although I would warn that it's real easy to misjudge how much space you need and end up with a completely full truck that's only got 80% of your stuff in it. Better to sometimes err on the side of too much space.
About having empty space in the truck... The way to pack a moving truck when there is a lot of space left over is to actually pile boxes on the ground so they take up the entire floor area and use other lighter packages to fill the voids. I learned this from a Fedex ground driver who had been using a moving van temporarily when his truck was being repaired.
I found this link for you that explains in greater detail
How to Pack a Moving Van | eHow.com