This is a good question. As you mentioned there are a lot of documentation examples available for OH2 which are not available for OH3. So it will be easier to learn OH2. Also if you updsate to OH3 there is not a big migration needed. Most of your rules will still working (if you are not using special features like Joda Time for time calculation)
But from my point of view I would start with OH3. This is much more user friendly and allows you to configure nearly all options inside the web interface. Also there is a new feature “Model” wich allows you to create a semantic model of your house. This model will build some location and equipment pages out of the box for you. If you decide to move later to oh, you need to build the Model later and this might take some time.
Also if you want to write rules you will love OH3. It has support for JavaScript out of the box. Also there is a new rule designer BLOCKLY which allows writing rules by drag and drop some operators. So for small rules here is no deep programing knowlege needed for defining rules.
If you decide to change to OH3. Do not use the milestone 4 release because this one was buggy and should not be used. Use the Milestone 3 or Snapshot release instead