Back on topic. For most things, most people will do, most of the time, an 8 core 3700X is fast/good enough.
But, the price increase to get a full 12 cores (3900X), has become so relatively small (12 cores, use to cost a huge fortune), that it could well be worth it. Even if it only occasionally helps you, speed wise.
In the UK, it is little more than a £100, to go from an 8 core (3700X), to the 3900X. I would imagine, that equates to around $100 to $135 more, in the US.