Twitter is no follow allthough it may provide some extra traffic I don't believe its very benieficial unless you already have a large customer/fan base.
lol there is no comparison to be made. Twitter is a nuclear missile and identi.ca is like a mini firecracker.
I've never heard of Identi.ca in my mind I'd use twitter, however you might as well get your name space in the other one, just to squat there for a while. AND if you can tie your twitter to your blog, your facebook and to your Identi.ca then you might be on to something. Corey