Absolutely, go for T&G.
You need a union, no doubt about it and they are, from my experience, better at getting things agreed by management. It sounds like rostering is one of your biggest problems. Tackle this and they may stop people leaving. A rigid set of terms and conditions will also stop you from being rostered back to back to backs and ensure you have adequate rest at home.
You also need to consider things that are personal to each individual, such as disciplinaries. You could be falsely accused of something by a passenger who doesn't get their way or another crew member who thinks promotion requires grassing up colleagues and you need someone to fight for you.
I have been down this road personally and was well supported, managing to keep my job and seniority. Other people in my situation were represented by other unions and didn't fare so well.
They will also protect your promotion opportunities, ensuring wherever possible that senior crews are recruited from within and not directly from outside. With the CAA's "one year rule" this may not always be possible but they will have to at least try to recruit from within.