The McLux TK comes with a flood reflector so it has a much wider, more even beam than the LSH-P but with quite a bit less throw. You can swap out the reflector for an NX05 and then the beams will be more similar, though the LSH-P will still have a bit narrower beam because of its high dome emitter. Total brightness should be comparable: the LSH-P comes with a P or Q bin high dome emitter and the McLux TK's (I believe) have a low dome Q3L.
In terms of handling and use, they're really not similar lights. The Arc is a compact, military-looking light with almost no concessions to style and not much modding opportunity. The McLux is quite a bit bigger and more configurable (modular); it has a pocket clip and threaded accessory ports; you can swap heads, battery packs, reflectors and optics, etc. It also has a striking visual appearance, an artful design that looks like a tiny Star Wars light saber. The McLux is not as easily EDC'able as the Arc, but it makes more of an artistic statement and is as close as it gets to poetry in a flashlight.