Should I use Drupal or a Mediawiki to create a site like Wikihow?
You have the source code for Drupal, as well as for Mediawiki (that's the nature of scripting languages such as PHP). This means, both platforms allow endless customization and you can implement your requirements using any of those systems. A decision between Drupal and Mediawiki is therefore not a technical decision, but a business decision: which platform will get the job done with lower costs?

If your are working on your own, you have to weigh "getting it done using something you know" against "risk using a new system but allowing you to become proficient in it". There are also other factors that come into play, like time constraints and the pricing scheme.

A correct answer to your question cannot favor one or the other platform. It can only give you suggestions on what factors to take into account so that you can decide for yourself. My list is not exhaustive.