Ironically, if it's a student card, the decline reason may be that they're unable to verify your student status. Many schools don't verify enrollment over the phone, which is how the CC issuers usually check. Usually this can be rectified by faxing or mailing a copy of a tuition bill or student ID. But it sounds to me like your credit is strong enough for a regular United card... why not try that? 1mi/$1 > 1mi/$2

Also, most issuers will let you convert cards to different types, so if you want a mileage card, you may be able to convert your existing Citi credit line to an AA card, Amex to DL, or BofA to US or AS - if any of those programs are of interest to you.