WordPress MU

I'm wondering which is best/ what's the implications of using either. I see default install is subdomains and see many MU sites using subdomains. Searching the forum drmike mentions it's an either or choice - but how/why did you choose?

thank you

"how" - can anyone please answer "how" do I set up things to enable subdir mode?

a lot of plugins i've tried to use won't work on subdirectory. i'd recommend subdomains.

As for the "how" = it's a radio button on the install screen.