In general, the answer seems obvious to me: it's better to have a $3/$4 card now than $3/$4 to spend later. If it's not, I'd question your choice of $3/$4 card.
The extra card reduces your chance of a turn 3 Overlord from 45% to 41% and turn 3/4 from 91% to 83%, but that's probably a price worth paying.
Note that gaining a cantrip on turn 1 has no detrimental effect whatever on how soon you might draw Overlord.
Gaining a Cellar, Guide or similar on turn 1 would be an utter no-brainer.
The big reason I can see to buy Overlord on turn 1 is what you're intending to use Overlord as. You might want to be out of debt before you play it, if you're going to play it as something that helps your purchasing.