Your choice, but if you're new to the game then I'd recommend just sticking with the class they come with. And Larian certainly say that multi-classing is better for more experienced players, so you might be being a bit too ambitious with a cleric/sorceror multiclass out of the gate!
That said, I believe you wouldn't get any more spell slots even as a single class, and it is the case that especially in the early game you need to ration your use of levelled spells if you want to have a reasonable adventuring day. Remember you can use scrolls, cantrips, weapons and grenades to save your spell slots.
Oh, and I'll move this thread to the Builds section where it possibly fits better.
