Hari Om!
Both versions are fine. As you had noted, ओम् clearly spells out the way the word is pronounced. ॐ should not be read as Oom even though it looks like that. It is a shortened symbolic version and that is certainly acceptable.
It may be of interest to you that there are two versions in the way that people write ॐ in English. Some write it as Om to spell as it sounds. Others write it as Aum, to spell out very clearly the three letters that make up pranava or Onkara. You must know the pranava mantra is only ekakshara, monosyllable even though it is made of three letters. Therefore Om is the more appropriate usage.
To answer your question directly, both ओम् and ॐ are acceptable.
