Let me put it simple. Xenoblade Chronicles = big game; Xenoblade Chronicles X = fucking colossal game.Oh guess I wasn't clear, I already own the Wii Xenoblade, it's just been sitting around since I'm never able to find much time to play many big open world games. So I'm wondering whether to play this first or get X for the Wii U and play that first.
Since you mention you don't have much time, I'm going to automatically recommend Xenoblade Chronicles (and you already have it.) This is a RPG that will take 80-90 hours with completing the most basic quests and leveling high enough to beat the game.
Recommending this game is hard because it's the type of game where if you don't play for a few days, consider your sense of story gone. It's something you'd have to play everyday, even for at least 1-2 hours to continually grasp the story. The Xenoblade Chronicles Wikia will also become your best friend to remind you what's happening. The story does hold significant and intriguing points, don't get me wrong. I really enjoyed the story, mythology, species, and lore on and about Mechonis and Bionis. Story aside, the combat mechanics were fun to learn (in my opinion) and the huge map environments are jaw-dropping to those who haven't played the game. However, it's easy to lose track with all the side quests implemented into the game. Some players find themselves dwelling in side quests completely forgetting what's even happening.
Above all, Xenoblade Chronicles and Xenoblade Chronicles X are not story related. You can play one without playing the other. I will mention, however, that Xenoblade Chronicles X has many nods to Xenoblade Chronicles. Regardless, it's not mandatory to play one first.