I get the impression these days that TC and Mashable aren't super psyched about covering startup launches anymore unless said startup has a pre-established relationship or part of their sponsored events.
If both of them give you the cold shoulder, I'd definitely go for ReadWriteWeb. I'm always impressed by the quality of journalism over there, and I feel that they really value their community and staying true to their focus.
And that "TC won't cover you" stuff, I dunno. If you are getting a lot of buzz, they'll have to cover you eventually. Who's to say either TC or mashable will even honor your exclusive? Chances are they won't even shrug until someone else starts buzzing about it at a lower level. Unless you have a guaranteed home run, I'd play it safe and hit up individual writers, build buzz from the ground up.