I have the Olight S1A (selected with the help of advice from the good folk on this forum!), and I really love it. I don't have the LD12 so can't comment on it or even say which is better.
What I love about the S1A is the floodlight beam, which has a pretty good throw. When I walk on some of the unlit private roads in my neighborhood, it lights up the entire road for quite a distance. And this is with an NiMH battery, not lithium. Great for walking the dog.
Another thing I love about the S1A is moonlight mode, or as I call it, the burglar mode. It's great to have a weak beam of light when you get up in the middle of the night and just want to avoid bumping into things. I studiously avoid turning on a room light or lamp when I get up in the night because light can stimulate secretion of the hormone that wakes you up. So a dim light is best for me for a trip to the bathroom when it's dark.
I also love that it remembers the last mode but still has the ability to turn on the light directly in moonlight mode even if the last mode is one of the brighter modes (simply requires a long push on the button).
I'm not crazy about the clip but it's useful if you want to clip it to the bill of a baseball cap and have the light shining forward. Helpful for grilling after the sun sets.
The magnetic tail cap seems useful but I haven't had a need to use it yet.
My one complaint about the light is the switch does not protrude very far, and it's hard from me to find it with my finger if I don't have the clip on. With the clip, no problem as I can orient the light in my hand using the clip. But without the clip and in the dark, it can be hard to find the button to turn on the light! With gloves on, even if I can see the button, I have a hard-to-impossible time turning on the light, and I usually have to take off a glove to do so.
I may affix some kind of highly protruberant bump 180 degrees opposite the button so that I can find the button more easily without the clip (though that won't help with the glove problem).
Overall, I am extremely satisfied with the light.