* Subdomains: some-team-name.app.com
* Directories: app.com/some-team-name
Subdomains were all 'hot' 10 years ago, but these days people seem to be comfortable with having a path in the url.
Subdomains look a bit more consistent (going from the 'small' to the 'large', user domain tld) than pathnames but that's personal preference on my part.
If you are going to use Google Analytics then you'll be pleased to know that if you use path names that Google will not get confused when counting uniques (using subdomains there is a lot of double counting).
* Custom domains - if you ever want to allow custom domains (ala Tumblr, Posterous, etc), it's probably easier to go w/ subdomains. The URL recognition will be much easier.
* Security - if you ever want to allow custom HTML/Javascript, I would say that custom domains are a must for security reasons. However, passing login credentials across multiple domains might be trickier than using a single domain.
* SEO - I'm not completely sure, but very curious to know. Can someone chime in on SEO tradeoffs of these two options?
I personally prefer subdomains, but I agree that people are generally comfortable with whatever approach you take.