First of all, I do highly encourage you to sign up for the MP Visa, if for no other reason than to get 21,000 bonus miles after making your first $250 in purchases. The first year is free, just be sure to cancel before the year is up if you don't want to pay $60 annual fee after that.

In addition, you would earn 1 mile for every dollar spent (including UA tickets). In other words, no "extra" bonus in using the card to buy UA flights vs. any other purchases. For this reason, it really doesn't make any sense to use the card for UA flights if you have other cards that give you benefits you value more than additonal miles. And if you fly a lot, the extra miles from CC purchases may pale in comparison to miles you get by flying.

How about your hotel stays? Can you use your own card for those? If so, I suggest looking into the card(s) connected with Hilton HHonors, Marriott, and/or Starwood, if you frequently stay at any of those hotels.