That was one of the first things I did was to change the default sms app from google hangouts, it confused the hell out of me, and it kept flipping between screens, did not like it at all. They should keep them as separate apps in my opinion. Also if you are sending like a thousand messages a day using hangouts, well maybe thats an exaggeration, but I do send a lot, I don't want to run the risk even though it may be small that messages get sent as a paid sms instead of a free hangouts message.
I have used gosms pro, and it does the job, but there are probably much better solutions out there. I also hate how they force you to use google plus to send images via hangouts.
They just take every opportunity they can to try and force you on to google plus.