So, here are my pro tips:
1. If you're not writing/blogging yet, please start. If you can start with sub-directories without making sacrificial offerings to your IT person or webmistress, go ahead and do it. If using a sub-domain gets you going, JUST DO IT.
2. If you ARE blogging already, but you're doing so on a sub-domain, you might be losing some SEO mojo. But, try and figure out how big of a difference this is going to make and whether it moves the needle for your business. If you think it's potentially significant, look into how hard it would be to switch to sub-directories. Often, it's a matter of setting up a reverse proxy and fiddling with your CMS/blogging tool. It can usually be done -- but tread lightly.
And remember, Matt Cutts from Google has stated in the past that it shouldn't matte whether you use sub-directories or sub-domains -- do what is convenient. Experts like Rand seem to agree that this is not the case right now. But, my guess is, eventually, this is going to be true (and going to get more true over time). Reason: Because it's the rational thing to do.
Oh, and here's the chart I promised: