T59, they are godly, a few of the other premium tanks are a bit out of balance as well. That's what we get with this new "free to play" but "pay to win" game models.
I'm a bit worried about the "real-time" changes. Managing mercs without turned based combat could be a disaster. Hopefully their support for turn-based combat will be intact.
The X-series are more like "versions", so really, the latest is usually the best to play. I would say you're not missing much, and gain a better interface and tweaked engine.