Soul Transfer allows both options without Enchantment and Artifact in play
While playing in limited format, my opponent cast Soul Transfer and was able to select both modes while only controlling an artifact (they had a Undercity Scrounger and some treasures in play). They controlled no enchantments and when I asked them to point out the enchantment, they conceded that they didn't have one.
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paintcan#67082 commented
I played a match where I had both modes available and conceded after I illegally used them. Here's a screenshot from later in the same match - I don't have the context menu popped open, but I have nothing on my board and all modes of soul transfer were available. Maybe it was counting enchantments and artifacts in my graveyard ?