Sending a text message through a arduino wifi shield
Sending an email you need a network connection of some form.  Options include:


Ethernet Shield
WiFi Shield
GSM Shield (can be expensive, but will work almost anywhere)


To send a text message you need access to a text message service. That can either be directly through a mobile phone operator or through an on-line SMS gateway service. For an on-line service the options are as above, since it's almost the same as sending an email (in fact, some gateways offer an email-to-SMS gateway option). For direct mobile provider access you can only use a GSM shield.

So really it boils down to how mobile you need it to be, and how much you are willing to spend (both on the initial hardware outlay and service provider fees and charges).