YMMV, but in my experience twitter would be prompter than the typical SMS service from a big carrier. More reliable, too.
Are you arguing that twitter-to-sms (not sms-to-twitter, which i have never experienced lag with) is a quicker trip than sms-to-sms? My provider is Verizon and my experience is VERY different than what you assert.
And this is coming from someone who has completely bought into the "OMGTWITTER" phenomenon. ;)