When rating the watt density it's based on watts/surface area, so w/sq in. You need to account for both the element's length and it's diameter. LWD/ULWD will be thicker as well as longer, which is why they're usually folded back on themselves.
From what I've read a standard density element like you linked will be fine. I wanted the extra assurance of one that's advertised as being able to withstand dry firing, so I went with the Camco 02933. It's the 5500w ULD that's NOT RIP. It's listed down a bit on the same page you linked under "customers who viewed this item also viewed." Only done one beer with the new rig so far, but it worked great. I'm sure either one would be fine.