Apart from the open source programmability that you've already mentioned, the other differences between the UltraFire C2 and the HexBright Flex that are most apparent to me, are in:
- The (amount of light cast)
- The likely of each (its resistance to damage & wear)
- The (the in-hand feel, the way the components fit together, and the look).
The info I found on the C2 indicates it's capability is between 250 and 300 lumen max (the various specs found were in disagreement). The Flex comes pre-programmed with a high of 500 lumen, but can be re-programmed to emit more than 650 lumen (I'm not sure of the actual max) at the cost of increased battery drain.
The body of the C2 is assembled from multiple pieces that screw together, and these pieces seem to be made of a somewhat thin, light aluminum. The Flex is a uni-body assembly (one piece) and is made of thicker aluminum.
With a uni-body having fewer points of failure than a multi-piece body, the structure of the Flex is already inherently more rugged. And when the added thickness of the body material is taken into account, it can clearly withstand much rougher handling & use without denting or cracking—better protecting the fragile internals.
The end-cap (closing the body and housing the button) on the Flex is the only externally separate piece and probably shouldn't be considered part of the body, since its only structural function is to seal the end. Despite this, you'll note the end-cap was also made nicely robust.
Other aspects of the design not already covered include:
The Flex has less contour stepping (diameter changes) across its length, providing a more consistent and firm grip. Its profile also allows it to lay still on flat surfaces and to easily be strapped or attached evenly to another object. In contrast, the C2 is more likely to roll on flat surfaces, and when strapped or attached would be prone to shifting unexpectedly or facing at awkward angles unless additional efforts were made.
The long and durable
The gripping surface on the Flex comes from large-scale ridges and hollows along its length and a faceted cross-section (derived from a hexagon, with the points clipped/chamfered), and so bumps, scrapes, and dents don't harm the grip. The C2 has a cylindrical cross-section, with much of the grip coming from the 'cross-hatch' texture cut in to the surface. This surface texture can easily be compromised by bumps and scrapes that don't noticeably damage the body or even dent, and over time it will also wear smooth in patches with normal use—from my experiences with flashlights and other metal items using similar surface-based textured grips.
Compared to the UltraFire C2, the HexBright Flex has a matte (non-shiny) finish, is roughly symmetrical across its length, radially symmetrical around its long axis, and has a simpler & cleaner profile while still being visually interesting. Obviously this last element of the design is almost entirely a personal preference/taste matter, but for me the Flex has the clear advantage in the looks department.
Regarding the Surefire R1 Lawman to which you linked at the end:
While the Lawman has a much higher maximum brightness (750 lumen vs. 500 lumen) with a longer battery life at that maximum (1.75 hours vs. 1 hour) than does the Flex, they're in completely different price categories (Lawman @ $455 vs. Flex @ $99 pre-order / $129 retail). So, to me the only sensible comparison to make is one of value per dollar. The result is that with the Flex you get 2/3rds of the brightness at 22% / 28% of the price of the Lawman. Or, to put it another way, with the Lawman you pay $0.60 per lumen vs. $0.20 / $0.26 per lumen with the Flex.
I hope that this was helpful and answered your question (and hopefully wasn't way too long or detailed of a response).