This has been discussed a lot.... it's alright though, you can ask whatever you want
Here's a thread on it that was made on the doc forums. It's pretty old but it covers pretty much everything.
Oh and ignore the title... Doc > Mario not the other way around
Seriously though... fundamentally they are the same but doc is more about defensive kill power and mario is more about combos. In the end it comes down to playstyle.
Make sure to read the green wall of text
http://smashboards.com/threads/why-mario-doc.264016/
And if you pick doc... welcome! You made the right choice
If not it's ok... I won't hate you... well maybe a little... nah jk... play whoever you want. In the end it comes down to who you have fun with.
I was talking to AyRookie (mario main if you don't know him) in twitch chat on Ken's stream and I asked him why he plays mario. He said it's because he feels like himself when he plays mario compared to say when he plays fox and I completely agree with him. When I try to play other characters it just doesn't feel the same as playing Doc.