I am really liking the MailChimp. I'm just on vacation and that was my deck of choice. But if I was choosing a charity to support, personally I like the charity water. Just my personal opinion.
Stack them up here:
Both are $9.95
Both proceeds go 100% to charity
It really comes down to back designs. I prefer the Charity Water deck due to the simplistic back design and I don't really like the ape/chimp court cards in the Mailchimp Deck (little to wild for me).
I definitely would choose the MailChimp deck, I love the back design and box, along with the court cards. Ultimately though, the decision is up to you.