There are two things to consider with sending photos. The size of the picture and the data it takes to send it. If you send a photo using a text message it is scaled down to reduce the size and data "footprint". This is fine if you are looking at photos on a phone (or even an iPad ) but if you want a better quality image to print, send it as an email with a larger (i use largest) file size.
As for data in text messages, using iOS messages is a bit different than a traditional text message. Messages will try to use a data connection first. If your phone can't access the internet (through a WiFi connection) it will revert to a triditional cellular text message format (notice some of your outgoing texts are blue, and some are green? Blue means it sent via WiFi (or the internet), green means it went as a traditional text message)
Depending on your carrier agreement, you may have charges for photos sent via text, or you may have a limit per month.
My thought is to always send a photo in the largest format in case someone wants to print the picture. If this isn't a concern, then go with a smaller file size or send is as a text