I think both sides are about equal in terms of numbers now, although I'm not sure. It's possible Amaok still needs more people. They were having trouble getting enough votes together in the past, I'm not sure how they're doing at the moment.
As for why you should join either side...
Amaok leads in terms of territory, although they recently suffered serious losses in orbital combat. Their rifles, although 'conventional', provide excellent long-range fire, devastating Moerth's troops in wide open spaces such as A2. Their fighters, although technically capable of serving both in orbit and in atmosphere, appear relatively ineffective compared to Moerth's spaceships. Allegedly, Amaok is ruled by a malevolent AI.
Moerth skipped conventional rifles and went straight for 'bolters', guns that fire micromissiles. These tear through Amaok's forces at medium range, whilst their accuracy drops off substantially at longer ranges. Moerth's destroyers are renowned for their sturdiness, easily tanking several missile strikes, although their manoeuvrability leaves something to be desired. Moerth has publicly stated an interest in genetic engineering, although no signs of this have appeared on the battlefield so far.
There may be other technologies operating behind the scenes that are not public knowledge, which may or may not be interesting.