Solr and Elastic search are basically sisters to be honest – both are pretty similar, with Solr being the older one. They are layers built on top of the core search library Lucene.
Is it an important decision in the first place? Can I easily slide between them later? It is an important decision and it is not easy to replace one with another. As you will shortly see, it is not that one is superior to the other, each of them has proven to be better at a particular problem than the other. Hence understanding your specific need is more important to make the right choice. Since it is a choice you make mostly once, I would recommend you spend quality time analyzing your use case.
Do you care more about ease of setup or the capability to tweak the relevance? If you are for ease of setup, go with Elastic Search. It is much faster to set up and it would give you a decent start.
If you are an advanced player who would want to be in full control of the search ranking (or relevance), then you have to pick up Solr. Not that it will give any better results than Elastic Search out of the box, but it would give you more control to create what you are looking for when it comes to ranking.
If you want to search for eCommerce products, documents like pdf, websites, etc, or any other search like music, that is client-facing, you are better off picking up Solr. You will spend some time setting it up, but you will not be later stuck with a search you cannot improve to full potential.
But when it comes to setting up a search for a Magento store, even considering Magento 2 Elasticsearch vs Solr is overkill. It is recommended that you go with a search extension like this one to keep the development cost at a minimum.
You can read more about Solr vs Elasticsearch here.