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The nice looking reflectors are of the same design, with the 'C' of course being larger diameter, and the 'C' appears to me to be a bit deeper as well. I would describe them as being 'high-gloss finish w/ light orange-peel texture'. Subjectively, I consider the resulting output radiation / distribution pattern to be essentially 'ideal' for general purpose, all around use. The outer border of spill is defined enough that I don't feel I'm wasting lumens and lighting the world, the near-field illumination is very good for close-medium range work, with enough central zone definition / concentration to also add some extra candelas and provide very useful / usable reach. IOW, a good all-around compromise setup. The SST with its apparently smaller die does make for a slightly tighter, smaller central spot, as one would expect. Personally, I wouldn't change anything, nor would I prefer an alternative optical element in these specific lights (although I do like some of those designs in other lights).
The control electronics differ between the 'A' and 'C'. I will try to confirm this stuff (for my own interest as well), but just from my feeble memory: I believe that both lights feature 2 user-configurable, persistent settings / options. Both provide an option for setting / selecting the output level for the lowest 1 or 2 levels in the normal sequence. Both provide a second option for setting / controlling the startup output level. In the case of the 'C', one may select any of the levels in the normal sequence as the startup level. In the case of the 'A', the option is to either enable or disable the 'last-state power-on/startup' feature. When enabled, it always starts at the level it was in when turned off. When disabled, it always starts at the 'Min' output level. Of course with either light, if one wants it to start at 'Max', setting the head to 'tight' does that. I believe the 'C' also has superior design for thermal management / regulation, which is highlighted in the product page descriptions on the IG site.
Keep in mind there are shinier, heavier stablemates D3A&C, which may be very different from these D25 Al models. I can't comment on those, but am aware of them.
Keep in mind that these lights were designed for minimum form factor of their type - with AA & CR123A cells. This means that only the shortest of the protected lithium ion cells on the market are short enough to work in these lights without spring over-compression, which must be avoided. It's not the end of the world, as the cells do exist, but it's the P1 consideration when shopping for cells to use in these lights, so be prepared for that. No big thing, but good to know.
ET's corp. documentation is unfortunately in a somewhat disorganized state. If I find functional links to some of the good info I know exists somewhere (which I've seen), I'll post them here. There's a thing or two I'd like to review for myself if I can find them.