I think most people would consider the Chase Sapphire better as an all purpose card. The points aren't limited to United, so you have more flexibility, plus access to the Ultimate Rewards mall. Sapphire also gives you double points on all travel spend, while the United card is only on United spend. Advantages of the United card are priority boarding and a couple club passes...not really worth it in my opinion. Biggest advantage however is the the United card gives you the points after first spend, while the Sapphire has a minimum spend of $3k. So if you're in a hurry to book, that can make a difference.

Assuming your credit is good and you don't have a ton of recent history or Chase cards, they'll usually approve both cards. You'll have to call, but it's not difficult.