Having been first on both sites, I can offer a bit of clarity.

Go with Techcrunch if you want to have VC's and other business's take notice of you. Go with Mashable if you want lots of users to read about your product and actually use it. Techcrunch is written for people in the biz. Mashable is written for people who enjoy using the tools biz guys make.

Also, Readwriteweb is a great blog and will likely still write about you even if they dont get the exclusive. Just make sure you save some info thats "just for them", So dont give the whole kaboodle to mashable/tc.