I'd get the LD01 unless you want max possible brightness in a small light. The LD01 is better built, thinner, has more battery flexibility, is and has (IMO) a superior user interface. The mini-cr2 is quite a bit brighter in high mode but that's about its only advantage. The LD01 is still pretty bright. They probably have comparable amounts of throw: the mini-CR2 puts out more lumens, but in a wider beam. The mono cr2 is thicker than the LD01 and its battery has about the same amount of energy as a lithium AAA. I have an LD01 and a Mini-123 and am not that tempted by the Mini-CR2. The Mini 123 is just a tad larger than the Mini CR2 and has 2x the battery capacity and it's much easier to get discounted 123 cells than CR2's, and a lot more flashaholic lights use 123's. IMO all three of these lights are too large for a keychain. I carry my LD01 in a Levis watch pocket and a Photon II and LPK on my keys.
The LD01 PWM is detectable if you shake the light up and down in low or medium mode, but I think it is improved over the older L0D. In practical use I don't notice the PWM.
I vastly prefer turning on in medium mode, but people's preferences are different and can only be learned from your own experience of actually using the interface. Simply working through the concepts in your mind doesn't count as much as actual experience. For example if I'm dark adapted and medium is too bright, I can either put my finger over the bezel while getting to the low setting, or just aim the light in a direction so the hot spot isn't directly in my field of vision. In practice medium is a very comfortable level the way I use the light. I'm not saying your preference will be the same, but if you haven't actually used the interface, it's more informative to try it than to theorize about it. With the Q-mini I find having to multi-twist to reach medium or high is much more annoying than having to multi-twist to reach low on the LD01, just because I don't use low that often. Main purpose of low is for a long running area light, and for that, multi-twisting isn't a problem.
If you really like the Q-mini interface, you can of course also consider the Preon.