I understand the advantages of a self-hosted Wordpress (or equivalent) blog. But do you think the like/reblog/follow/social features of Tumblr provide a better advantage for spreading your posts?
The biggest problem with having a blog at a startup (or otherwise) is actually blogging, not the platform. Trust me, I have many a dead blog with carefully configured Wordpress installations.
Start with Tumblr. Write 10 high-quality blog posts, that get 100 readers a day. Then if you're not happy with Tumblr, think about changing.
If you want to engage people, make it as easy as possible for them to participate/contribute.
Any blog platform can use Disqus, so that opens up the participation. Tumblr has the share/reblog controls for Tumblr members, so they can share your posts easier. Any blog platform can have Facebook/Twitter/Google share controls.
The next problem is your identity. If you host your own blog solution you can have it on a url like yoursite.com/blog or blog.yoursite.com
Tumblr also supports custom subdomains, so you could use blog.yoursite.com
The next problem is uptime -- Tumblr frequently has issues with uptime in my experience. On the other hand, if you're having uptime issues with your own site, it might be better for the blog to be hosted somewhere else so that you can let people know what's going on
voting for 'doesn't matter one bit' with the exception that if you want to work it into your existing site, it might be a bit easier to have full control with self-hosted.
Start with Tumblr. Write 10 high-quality blog posts, that get 100 readers a day. Then if you're not happy with Tumblr, think about changing.