I use Textra because it has certain customization options that I haven't found in other SMS apps. For example, when I got my Nexus 5 hooked up MTS, I could send and receive MMSs, but not if I was on Wifi. Somehow having WiFi enabled would screw things up for me. I've come to the conclusion that when I sent a picture or receive one, the previous SMS apps would try to use WiFi to get the job done. While I definitely see that as an advantage, my carrier doesn't seem to support that. I tried out Textra on a whim, and as it turns out, Verizon or another US carrier had this issue as well and there's an option that when you receive or send an MMS, it will temporarily disable WiFi if you're connected.
Along with other neat features like making a noise when a text is sent out or choosing how long to have a text float on the lockscreen is part of the reason why I prefer Textra over the rest. I will admit that I do like the UI of Google Messenger.