Before I begin an extensive search, are there important differences between Google Patents, Espacenet, The Lens and PatentScope; or is it a matter of usability/preference?
Yes - there are differences amoung these and others. The number of fields you can independently search with google patents is very limited and they are thought of as missing some recent documents. Unlike many others they do not have a way of setting up an account where you can save your work or any analytics. The Lens and others can give you charts of what inventors and applicants, assignees or classifications are the most prolific in a set of search results.
But google patents is always improving - I noticed a selection criteria today - "litigation" that I had not noticed before. And they are very forgiving of misspelled search terms, just like Google searching.
I just searched for patent documents from a large family of patents I filed for a client. With google I got two hits, one for a U.S. patent and another that somehow represented many others. In The Lens I got 31 distinct hits.
Although it is only U.S. patents, the USPTO site has very fine grain searching of fields. I wanted to find U.S. patents with assignees in my city of Long Beach, CA. Since I stopped my subscriptions to expensive paid patent search sites I looked in the free ones and no other site had the ability to narrow down by assignee city.
PatentScope and Espacenet are similar, they both allow creating a free account. like the Lens for saving your work. They allow downloading results as a spreadsheet. I just noticed Espacenet offers downloading results as a single PDF of all the first pages. If the primary drawing is likely to be helpful this is very valuable.
One thing to look for are the set of boolean operators a search offers. Some offer and some do not offer a "NEAR" operator. Many documents might have the terms "wire" and "a mile long" but you might be looking for one where the two terms are within the same sentence or paragraph or within 10 words rather that 20 pages apart.
I would advise learning the tricks and capacities of one very well but also double check you haven't missed something by using at least on other.
I hope others post more comprehensive and complementary answers.
As an aside to George Whites excellent answer, if you know the patent or application number, Google Patents is generally the fastest way to the document. That said, I greatly prefer The Lens. When searching my own name on Google Patents, I have trouble getting all my patents listed. The Lens always reports them all. I think this has to do with Google Patents grouping by patent family, but either way it's annoying.