Take one minute of video. Look at the file size. Now how many of these 1 minute files can you fit onto an 8GB card in your particular format and camera/device.
SMS stands for Short Message Service, which is the formal name for text messaging. It's a way to send short, text-only messages from one phone to another. These messages are usually sent over a cellular data network. That's not always true, though, as in the case of iMessage below.
For starters, SMS messages require a cell phone service, which can be really annoying when you don't have it. Even if you have a full Wi-Fi connection at home, school, or work, but no cell service, you can't send a regular text message. SMS is usually lower on the priority list than other traffic like voice.