Don't forget to shop around for the best deals for enrollment. Right now, UA is offering 20K miles plus a $25 coupon and, I think, a 1,000 mile upgrade cert. This should factor into your decision as well. Additionally, you can hold up to 4 UA CCs at the same time and should expect to get the bonuses from each one, so it's a pretty good investment. Many folks sign up for the cards, make a single purchase, then cancel the cards. Not sure just how this affects credit ratings (probably has no real effect, IMHO) but I can tell you that many will do this.

You should also consider whether you can get anything in the way of qualifying miles for purchases. UA's currently advertising a platinum card where you get 5K EQMs for signing up and 5K EQMs for spending $35,000, plus companion certs and other stuff.