I became interested in NeuronWriter after I realized that it's world-friendly (one of the only multi-language SEO optimization tools). This is quite a big deal, and what got me over the start line.

I've been using and testing it for a while and I must say that it's the closest to Surfer SEO that I've seen - focused mostly on the optimization aspects.

While I've had it, it has undergone many improvements and the new (useful) features keep coming.

I think that Tier 3 should be the only option, as it has all the features - lower tiers are missing some features, so just be aware.