What's stronger: phone/text 2FA or authenticator app?

I noticed some very advanced sites don't offer 2 factor authentication via phone/text. Example Salesforce's Heroku: Is phone/text based 2 Factor Authentication generally weaker than using an authenticator app? I'm trying to work out why a major site would not offer 2 Factor Authentication via phone/text, but offer other methods (authenticator app) instead?
http://dlvr.it/Rz3xwN