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I suggest Google Voice, as you can send/receive texts (and make calls) from your Mac as well as your iPhone. And you'd be saving upwards of $30 a month by not having a text messaging plan on your iPhone.
 
I use my Google Voice account for SMS. I can use the official Google Voice app on my iPhone, and either their website, or a Mac app called VoiceMac to send them from my Mac. It's a pretty sweet system, I never have to reach for my phone when I get a text now.
 
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