I could not recommend either way whether to hire a test pilot or not - it is a too complicated and variable decision and depends on the particular situation.
However, I will explain how and why I decided to make the first flight myself.
I tried to prepare myself with a thorough reading of all the available FAA first flight guidance and made a simple first flight 'before and during checklist'. Then and probably the most useful, I got several hours in friend's same type, same engine/prop equipage planes, including multiple landings, so I knew what to expect. And I flew the routine in my head countless times...till I could hardly wait to do it for real.
As far as the plane goes, it had gone through several fast taxi tests and fuel system examination by a very knowledgeable A&P. Finally, and what I considered most important, in my mind it came down to, did I trust my work and preparation, and if I did not or wasn't sure, why would I want to endanger someone else? Even if they are an "experienced test pilot", if something drastically failed or came apart, they too might likely not be able to do anything about it. Anything else, the plan was "circle back down to the airport". As far as flying the plane was concerned, it should behave like the other two I already flew - and it did! One minor electrical squawks - an EGT that didn't indicate, and that was it.
It's great that AC 90-116 now exists. At the time I did my first flight it didn't, but surprisingly enough my FAA AW examiner literally told me "What do you think 'the crew required for the safe operation of the flight' means?" I took the hint and after the first flight alone, took a qualified pilot friend along, I flew, he took data. It was one of the most memorable experiences of my life.