A subdomain blog with good pagerank also gives you the ability to add heavyweight links to content on your main domain when you'd like to drive traffic to specific areas.
Also, no disrespect to any other posters, but a new company/site blog likely has 9 months before it starts to earn enough link-juice to need to worry about spilling some...
Check out http://iwantmy.name/a for an overview of blog services that offer a custom domain option.
Otherwise, subfolder is either better (SEO) or a no score draw (everything else).
(Honestly, I doubt it matters much, but blog. is my [slight] preference. In any event, it isn't difficult to make one redirect to the other, and it's wise to do so, whether the Google juice the other answerers mention is important to you or not.)
My searchyc.com query was '.com/blog'