If you use your truck, bumper.
But, seeing as you use it only occasionally, and don't tow with it anymore, I'd go with the roll pan. I think they look a ton better (the bumpers stick out too far IMO), and, it really makes them sleek. And, if you continue the stripes all the way down, they look really good. Also, I think its cool that these trucks came with them stock...so, its another one of those special things that separate it from the normal Dodge Rams.
Also, I drive my truck everyday, and its never posed a problem. And, if I got rearended, I'd be in prison for the rest of my life for killing the person that did it, anyway, so...it wouldn't matter if there was a bumper there, or a roll pan...lol
Here's a pic of mine for some inspiration (don't mind the left exhaust tip being lower...my rear sway bar bent it :/ )