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Anywho...
Arc LS beam will generally be better than the CMG Reactor. Reason? Arc does a much better job sorting out their LSs, and thus, you get generally brighter, cleaner, more even, and more whiter beams than you would get from a Reactor.
There are the occasional times though, and very occasional, at that, where you'd get a perfect Reactor, with a nice, white, textureless beam that's just as bright, if not more than an Arc. But those are rare. And the worst thing about the CMG is that the LSs are hit and misses. Sometimes you get a great one that lights up your room. Other times, you wish that it would self destruct... Plus, lithiums are a necessity if you want the Reactor to be anywhere as bright as an LS is supposed to be; something the Arc doesn't need, but can take for longer runtime. With plastic versus HA III and the hit/miss game of the Reactor, sometimes, you'll just want to throw it like a grenade...
I couldn't readily find a link to a beamshot comparison, but if you really really want some screenshots, I can post them later on...