While I definitely think you should focus on fit when selecting a mountain bike, I tend to think you're overthinking the rest of it. You gotta start somewhere, and the Jett is a perfectly nice bike, as is the X-Cal. It will more than you get you started in the sport. I understand the appeal of higher end bikes, but don't lose sight of the fact that you don't have to buy high end to ride and to ride well. Get the bike and ride it. I'd add that from my own experience as a relatively new mountain biker, I tend to think that taking a clinic or some lessons will help you ride better far more than getting a higher end bike will.
I really urge you to hold off on swapping out anything for now. Would you notice the difference with lighter wheels? Sure, but you'll be in a much better position to figure out what you want if you ride the bike for a time first. Certain things on the bike are wear items, so upgrading from the get-go doesn't make sense. Just let them wear out. As for wheels, I generally think they're a good first upgrade, but I wouldn't put any money into a bike until you're really sure you like it and are sure what kind of riding you do on what kind of trails.
Live with intention. Walk to the edge. Listen hard. Practice wellness. Play with abandon. Laugh. Choose with no regret. Continue to learn. Appreciate your friends. Do what you love. Live as if this is all there is.
--Mary Anne Radmacher