First off, i have a collection of headlamps (ok, it's an addiction for headlamps). So, i own all of the ones you mentioned and those mentioned by others in this Thread.
All of the ones mentioned primarily break down, IMHO, into two broad categories - 5mm LED units and Luxeon units. This breakdown is independent of regulated vs. unregulated output.
I think you must first answer the question of usage. What do you want to use the headlamp for - nighttime navigation or merely as a task/proximity light in camp. Or Both functions?
Clearly, the Luxeon LED units (Eos, TikkaXP, Spot) will outperform the 4x5mm LED (Quad, Tikka, TikkaPlus, Zipkam, ZipkaPlus, TacTikka, TacTikkaPlus) in terms of light produced, but at a cost. The cost is battery life. The 4x5mm units can produce light for a longer period of time from the same 3xAAA battery source since they draw less current when they are operating.
If you decide to go with a 4x5mm unit, make sure that you buy a 2006 or later model year unit. Why? I believe it was only in the 2006 model year units that the new twice-as-bright 5mm LEDs were introduced. These really make a difference. In my side-by-side comparison testing with identical units from previous years (pre-twice-as-bright LEDs), with all units using fresh batteries within 0.05volts of each other in beginning of life no-load voltage, the newer units appeared to be easily twice as bright - the difference was really quite impressive.
Of all of the units mentioned, IME, i would stay away from the Spot. I had a first production run unit and it drained the batteries much, much faster than the Mfr figures stated. It had a better beam pattern than the Eos (the bright center "spot" had a larger diameter). IMHO, the TikkaXP had the best beam pattern of these three Luxeon LED units.
Search the Forums, i have posted several side-by-side qualitative evaluations of all of these units over the last couple of years. My "old-timers" is acting up so i hesitate to try to repeat all of my impressions purely from memory.
I like the Quad and Eos, but unlike the younger(???) Poster(s) to this Thread, i need to use medium, not low, to navigate a trail on a moonless night, but i'm old and so low-light vision is naturally degraded versus a younger person.
My favorites are the ZipkaPlus (same as TikkaPlus but with a retractable elastic cord instead of an elastic headband - makes for a nice tidy package - i have one with me at all times, even at work), and the TikkaXP (which i found superior to both the Eos and Spot). I really like the Eos too, but it's beam pattern is too small, making one resort to "chicken walking" with the head bobbing about trying to place the "spot" on rocks and trees, attempting to locate trail blazes. Still, there's something about the Eos i still like. The Quad is also a good choice for a 4x5mm headlamp, though side-by-side, my ZipkaPlus2006 is brighter than my Quad.
Regulated vs. unregulated: well, that's a personal preference. I like PTec's approach to use regulated output for as long as possible and then automagically switch to unregulated output. However, if you use alkaline batts, you don't get as long regulated output as you might hope for - particularly if you use high output (like only one hour regulated HIGH output if alkaline batts are usedm IIRC), which generally isn't necessary as medium suffices even for my old eyes. Li batts in the Quad and Eos produce much better results.
The BD Spot is unique among all of these units since it has both a Luxeon LED and 5mm LEDs, but my personal experience with it has shown it to be a "batt hawg" - otherwise nice, except for the sequential switch operation which is just a personal annoyance (some, however, do like BD's on-off-on-off switching sequence). For me, too much switching to get to the mode that i might want at a particular time - a lot of lighting mode choices though.
If i could have only one, i'd pick the TikkaXP AND the ZipkaPlus2006 or later model year. Wait, that's two, so...
TikkaPlus if nighttime hiking/biking was its primary use with some around camp proximity tasks thrown in,
ZipkaPlus if task lighting was the primary use with little nightime hiking. I don't think either would be appropriate for biking as 5mm LED units generally don't have much "throw" and if collimating optics are used with them, they make a relatively small "spot".
This is not to disparage the Eos or Quad. If you decide on one of those, you have also made a good choice.
My two shekels.
[Note: Please, don't anyone think from these comments that i'm an "all things" Petzl fan. Personally, for AA batt powered headlamps (not AAA powered as is being discussed here), i prefer the PT Corona (or the bike version, 8xAA batts like in the YukonExtreme which i've dubbed the "CoronaExtreme") and the PT Apex versus Petzl's offerings.]