'deem' is more formal than 'consider'. They have the same meaning, but in conversation you don't hear 'deem' often unless it is in something more formal like a presentation or a speech. In writing, 'deem' is a little more common. In formal writing - journals, etc it is common, but even in newspapers you will see it. As well if you were writing a college level assignment, you may see 'deem' written as an alternative to 'consider'. Often in this sort of formal writing the two terms are mixed to avoid repetition of one.