Cyberkid. @Groen. Founder of https://t.co/nZ7MJu2W7Q. (FYI, tweets get deleted on a irregular basis)
Mostly on price (they have a generous free tier) and the fact that they are a European company. Also there template language works great.
Adding not so much. The biggest difference is that it in the first place works on your local machine, so being offline isn't a problem, sensitive data aren't a problem.
Personally I use Mailjet using SMTP. For most services, plain simple SMTP works best.
As I didn't found a setup that worked for me I created Millikanban as a free tool. Just a little kanban board, nothing else.