couple of differences here.
Liners: the hot form will afford a longer "feel" to the fit once it has been "hot formed" at a dealer ( really do this if you get these boots - it makes all the difference in way the liner feels). it will also seem to hold your heel-area better. it's a great liner. the ultrafit liner is slightly lighter and will fit a little shorter. also a great liner. so, from a liner standpoint, if you need a little extra length, the hot form is the way to go. i downsized alot of people a shell size with the hot form liner in the icon series (the opposite is now the case with the diablo - easier to downsize with an ultrfit liner, or seems to be the consensus at least).
Shell: you can make a DPXR just as stiif, in theory, as the alu. but that means locking the cuff out. the DPXR can be made into a lot of boot from it's high-advanced base level. if your're super aggressive, the Alu is the way to go. if you're a little less on the aggresive side, get the DPXR.
knowing the boot you are getting out of would most likely be the clincher here.