I would hold off, I upgraded my ipad 3 to 8.4 and it lags badly. Can't get it to downgrade, always throws this generic error message.
Do a search on 8.4 and lag, many people are experiencing the issue.
As mmntech suggested, you cannot really compare your iPad 3 with the iPad mini 2. The mini 2 is way, way faster. The iPad 3 isn't even in the same league.
In your shoes, I'd stick to iOS 7.1.x for a while, and then eventually consider making the jump to iOS 9. Just skip iOS 8 altogether.
That said, I'm running iOS 9 on the iPad 2 already. Works OK, albeit not as fast as iOS 7.1.x. You don't get all the new bells and whistles but at least from an end user point of view you do get some benefits like more matched UI details etc., and more importantly, it's not any slower than iOS 8.x. In fact, Apple said they're aiming to speed things up on iOS 9 over iOS 8 on older hardware, and I'm optimistic they will because iOS 9 is already as fast as iOS 8.4, if not faster, and iOS 9 will likely get faster with subsequent updates.
P.S. Congrats on your 5000th post!