Praise be to Allaah.
Shaykh Muhammad ibn Saalih al-‘Uthaymeen was asked this question and he replied as follows:
What we advise is that he should start with al-Baqarah, unless he is more familiar with al-Mufassal. For some people it is easier to memorize from al-Mufassal because the soorahs and aayahs are shorter, and he often hears them from the imams in the mosques. If this is easy for him, then let him start with what is easier. We also advise him to pay more attention to consolidating what he has memorized than on memorizing a lot, because it is more important to take care of what he already has than what he hopes to gain.