I prefer X-wing, but the games are very different and it's almost a false comparison.
X-wing is a fast-paced tactical dogfighting game between fighters (some freighters may apply). The players can change plans from round to round, and knowing ranges, dials, and being able to estimate without pre-measuring are crucial to success. X-wing is fast to pick up and while new upgrades add a great deal of variety they don't slow down play.
Armada is a slower, more strategic fleet engagement game focused on capital ships (with fighters present, but clearly not the focus). Planning ahead is more crucial while precise placement is not as pre-measuring is allowed. I've found Armada harder to pick up (perhaps because I started with X-wing) and a bit more complicated in play. Games take longer.
I think they appeal to different needs and niches, but share common Star Wars flavor and great miniatures, plus some common game concepts.