Overall, the High CPU Droplets are designed to provide more compute power over the processors in use by our standard Droplet line.
On the High CPU, currently we're using the Intel Xeon E5-2697A v4 (2.60GHz) whereas we're using a mix on the standard Droplet line -- the 8GB I just tested uses E5-2650 v4 (2.20GHz).
For those who aren't doing CPU-intensive processing, the difference will most likely be negligible. In most cases, RAM and static caching is going to be more important than a difference of .4Mhz to 1GHz in CPU. That's based on experience working with some pretty high traffic WordPress sites (ranging in from 200-500k to 1-2m visitors a month up to 20m).
Throwing WooCommerce in to the mix makes RAM and static caching all the more important.
It also comes down to traffic levels and overall configuration and optimization of your stack. The key difference here is that WPEngine manages setup, configuration, and software optimization on their end so you don't have to (beyond WordPress -- as in Apache/NGINX, MySQL, etc). When you venture in to a self/un-managed environment, you then become responsible for those management tasks.
In such a case, on an un-optimized stack, you may find performance is actually worse than what you are moving from as default configurations are not well tuned for production. It takes tweaking and tuning to get a balance :-).