The Demon is much better than the Ravana at anything except going on the Offense.
This makes it contrary to widely recognized Shivan attack patterns and hence, the more (or shall I say, completely) offensively-oriented SD Ravana is used much more often.
I personally think that the Demon's anti-fighter capability is far better than the Ravana's.
Sure, the Ravana has a few more odd flaks, but it's SAAAs have atrocious firing arcs. Both sides are completely devoid of coverage. The Demon's have a fire arc enabling it to hit most fighters except for those attacking it head on (for which it has a blob, a flak, and a missile battery to deal with). The Demon's anti-fighter guns can also take a lot more punishment. The Ravana's spine SAAA is as woefully underarmored as its main cannons.
The Demon can however, be taken on the offensive. It's LReds have very good firing arcs (almost 180 degrees). As long as they jump in slightly to the side of their target, they can at least bring 2 beams (1 LRed, 1 SRed) to bear. However, it is more difficult to position it like that, and in the same situation, the Ravana can muster twice the firepower, making it seem more of a threat.