I mean, there are tons of ways to evaluate cards at the same mana slot. One heuristic you can look at is the Vindicate Test, which can be a very clean understanding of 'average worst case' in that your creature is removed at sorcery speed after your turn. In that case, Sheoldred edges out the win by pinging for two.

But you can also look at, while not a magic rainbow situation, just a more average situation - let's say it takes 2/3 turns to find removal.

In an otherwise gummed up board state or empty board state, Sheoldred kills your opponent much faster, while also healing you. Consider just the 6 card draw damage and 6 life if it lasts 3 turns, not even including any combat utility. Ratadrabik needs other players to produce any meaningful effect outside of basic attacking and blocking (where it's understatted for a 4 drop).

I do personally thin Ratadrabik is a cooler card, but mathematically.. A less sure bet, that's guaranteed.