Poirot and Marple are different in character, although the books that feature them were written by the same author. Where they 'join up' is in their treatment of the 'lower orders'.
Agatha Christie wrote her books between the wars, when British society was still fairly polarised. There were the aristocrats, the well-off and the pushy, the professionals and there were the workers who came cap-in-hand still. Professionals could mingle with the first three, but patronised the last.
Ms Christie gave away her background in her writing and her deductions were coloured by her background. Aristocrats, the well off/pushy and professionals murdered out of greed or revenge, the under-classes murdered for gain or out of frustration,
That said, I'd opt for Miss Marple as being more rounded.