Go with the DL3XX series, and avoid the DL1XX series like the plague.
Our 3XX's are rock solid and have great coherent design throughout. The 1XX's were designed by monkeys on nitros. We've got over 30 3XX's and all but one has worked great. We bought 5 from the 1XX series and only 2 worked out of the box.
Not only that, the ergonoics of the case designs are worlds apart. The 3XX's has an intutive use to use lid, everything is easy to reach and it comes with the on an only tool you need for it in the case. The 1XX's are hard to open and the case design makes it so you need a screw driver (not included) just to get the ethernet cable unplug from the the first ehternet port.
Seriously, I could go on for hours. I'm assuming the 3XX's come from HP proper and the 1XX's are from shitty hold over compaq group, but that's just my guess.