2. Ghost has SEO & social built-in
What’s the point in publishing if nobody reads your content?
Ghost has the best support for search engine optimisation and social sharing features of any blogging platform in the world. You don’t need any extra plugins or extensions, and you don’t need to write any extra code. It just works.

There are built-in XML sitemaps, Google AMP pages, canonical tags, optimised URLs, microformats, Facebook Open Graph tags, support for Twitter cards and clean semantic markup. All of this is done for you automatically, with no plugins needed.
3. With Ghost, you own your content
When you blog on Tumblr, you give up the rights to your content. They have the right to suspend, remove or repurpose your content as they see fit. All of your writing is contributing to Tumblr network. While this gives you the benefit of a network effect, where more people are exposed to your work, it means that you ultimately have far less control. Setting up a new Ghost blog is just as easy:

Because Ghost offers an official hosted service, you can have a new blog up and running in the space of about 3 clicks – with exactly the same level of control as if it were hosted anywhere else. Powerful automation simply saves you a lot of time.
4. Ghost is faster
Because Ghost is powered by a modern technology stack using Node.js – it’s fast. Really, really, really, ridiculously fast.

On top of that, downtime is limited by each blog being separate. If Tumblr goes down: All Tumblr blogs are down. If a Ghost blog goes down, none of the others are affected. It's far more stable and reliable to host your publication in a place where you know it's safe from attacks.
5. Allows you build a business around your blog
With Ghost there are no limitations on what you can and can't put on your blog. If you want to use display advertising, that's fine. If you want to collect email addresses as leads to drive to your business, that's fine too. If you want to create a platform around your blog to start selling your book, that's A-OK.
If you're looking to graduate from the simple social-network style blogging of Tumblr to more serious publishing as a career: Ghost gives you a lot more flexibility.
"We’re using Ghost’s Pro service to host the blog for us. Works really well. Great writing experience compared to WordPress. WordPress is an incredibly powerful framework but because it supports the needs of so many websites and demands, it has grown into a sizable and cluttered tool."
Switched from WordPress to Ghost