I use both. A Wordpress install for my public blog, and Posterous for a blog to share w/ friends & family during an internship.
I don't have any strong opinions here. Wordpress is great, and so is Posterous. The few things you may want to look into are:
- Updates: Wordpress, both the core and the various plugins, needs updating. Usually everything goes well, but still it's a minor annoyance.
- Plugins: As you said, Wordpress has plugins for about everything under the sun. :) Might come in handy.
- Ease of posting: Wordpress isn't that bad. You can even set up email posting like in Posterous.
- Ease of install: installing Wordpress on a vanilla server, though quite easy, is a nice way to brush up your admin skills.
Posterous offers a very straightforward Google Analytics integration. Just go into your control panel, paste the bit of Javascript GA gives you, and Posterous will display it in every page.
Ideally I'd want it to reside at www.domain.com/blog .. I'm not sure if I'd be able to do that with Posterous ..
I'm not sure either. You can definitely set up your own domain to use with Posterous though.
I guess in the end the biggest question is whether you want to spend time fiddling with your install or not. If not, then Posterous is dead simple and has everything. If you're ready to spend some more time, then Wordpress can't hurt.