I have the Elzetta ZFL-M60 3-cell model with Hi/Lo tailcap. It is an excellent light in the tactical category. Mine is an original model which included the Malkoff M60 drop-in module, which all the original Elzettas had.
I also have had all the Hound Dog lights except the new XPL model. It is also an excellent light, and I think it is just as 'bombproof' as the Elzetta, except it has a glass lens, where my Elzetta has the polymer optic of the M60.
In my case, both of these lights are actually Malkoff lights, in the 'light' part. They just have different bodies and fifferent Malkoff light engines.
IMO, the pros and cons are that the Elzetta needs to be bored to take 18xxx size cells, and the HD can accept 18xxx cells standard. Using primary cells gets expensive, and using 16xxx cells sacrifices a lot of run time. The Elzetta has a slightly smaller head, so it might be a little more pocketable, but it doesn't throw as far.
It's a tough choice, but if I had to grab only one of them in an emergency, I'd take the Hound Dog. I have a charger in my car, and it is the more capable light, overall, IMO.
My 2 cents.