I definitely vote poirot , I just love him and Suchet has been the perfect actor for poirot .
I am glad you are going to persevere with Miss Marple, There is a Magical Quality to most of the Books she appears in, I am currently re-reading 4.50 From Paddington and am loving it all over again
For me Miss Marple is only a bystander in Murder At The Vicarage and The Moving Finger which she almost just has a Cameo in, I know Poirot only comes in really late in The Clocks but in a few he comes in quite late and he is in too many where as the amount Miss Marple is in is just right although she shouldn't be in The Moving Finger and Probably shouldn't be in Murder At The Vicarage.
Detective Fiction shouldn't be Totally realistic should it? and having it totally realistic would spoil them, I thought having Miss Marple sent for to solve Mrs Symington's Murder when she couldn't have known every bit of what had been going on make The Moving Finger unrealistic and although she didn't have to come into the Village having her suddenly interested in Colonel Protheroe's death enough to solve suddenly was improbable but it doesn't matter because both are good books.
This question is so like Apples or Oranges? - They are so different and picking one over the other is highly subjective. I like them both in different ways. I love how Poirot is so puffed up and full of himself. Christie does a great job of letting us admire his great detecting abilities yet still laugh at his conceit and vanity. We can laugh at him yet admire him. Marple is like so many elderly " shrewd cats" that we all know in our lives who can noisily find out personal details and see right through phony people. The two have different investigative methods - Poirot very thorough and precise , Marple insinuating herself and gently probing around. Bottom line - Christie created two very distinct and successful characters and I enjoy them both!