Pros of 1.9: It's the newest version meaning once all the bugs are sorted out all players will have the most optimized experience. Hacked clients will take longer to update, meaning a hacker free game for a good amount of time. New PvP mechanics will come into order.
Cons of 1.9: Duel wielding is scary and strange, people generally dislike it. PvP mechanics are similar to those of 1.8, and 1.8s are a bit unfavorable.
Here's the thing. We can't stay on 1.7 forever, why not embrace the change? It'll be a bit weird, however eventually it'll become the norm and I'll bet everyone will adjust just fine and maybe even find things in the future we liked more than 1.7
Let's give it a shot, eh?