- Domain.com/blog might be better for seo, but it's probably not a big deal unless you are hoping to get lots of seo juice via your blog. - blog.domain.com is

- Domain.com/blog might be better for seo, but it's probably not a big deal unless you are hoping to get lots of seo juice via your blog. - blog.domain.com is easier to manage (easier to use third party service, easier to keep code updates separate, ...).