The difference between an SMS directory and accessing a directory using an app or by mobile website is the fact an SMS directory will return results in areas that do not have internet coverage.
Here is an article that emphasises how a local search directory is used so much by people on their mobile phones. It compares having an app against a mobile website. It doesn't mention an SMS directory as they do not have the technology (above) yet to be able to return texts of 450 characters. When an SMS directory is introduced I believe it will surpass apps and mobile websites due the number of people without smartphones and being unable to connect to the internet. Texting is faster and easier to use
Use of local business directory apps up 42%The total US local mobile audience reached 17.3 million as of March 2010, according to research conducted by comScore for the Yellow Pages Association (YPA).
That represents a nearly 14% year-over-year increase. Based on eMarketer’s estimate of 285.6 million mobile subscribers as of December 2009, the most comparable date in our forecast, just 6% of mobile subscribers access local business directories on their phone. But among mobile internet users, the proportion is much higher—more than one in four mobile subscribers who use the web on their phone at least monthly also use their device to search for local business information.
Between March 2009 and 2010, an additional 1.9 million mobile subscribers used browsers, the most common method, to access business directories, but growth in directory usage via apps was even steeper. Directory app usage was up about 42% to 4.1 million mobile subscribers.
A July 2009 survey from comScore and TMP Directional Marketing similarly found browsers were the more popular method to access local mobile content, but that app usage was growing more quickly.
“Mobile offers significant opportunity, both for consumers who need convenient and reliable sources of local information on-the-go, and also for local search providers that are making this content available in new and innovative ways," said Neg Norton, president of the YPA, in a statement. "Yellow Pages and other local sites that have a legacy for providing trusted local business information via print directories and Web search tools are best poised to take advantage of this phenomenally versatile and interactive media. Mobile allows them to extend Internet Yellow Pages to consumers wherever they are.”
