Fairline 65 or Princess 64?

Very close call. Since both have the same hull shapes of Bernard Olesinski and similar construction methods.

Fairline of 2010 or before tended to be that (very) little bit better then Princess of the same era. Usually in areas concerning interior finishings.
From 2010 onwards and the purchase of Princess from Hermes things have changed.
In most models Fairline usually have a slimmer hull to Princess which usually makes it ride better in head seas.
In this case the Princess 64 is actually slimmer.
The Princess 64 was in reality a development of the 62 extra extended up down hydraulic platform and redesign of stern with two entrance from platform to deck.

Have you considered a look at a Sunseeker manhattan, their 60-64' Manhattan is a phenomenal riding boat and I'd say construction a touch above both that you're considering.

For that age the Sunseeker he should consider is the 60 or 63 Manhattan. Which I think will again be a very close call.

What Sunseeker have better to Fairline and Princess is usually access in working areas, and engine room space is usually better.
As for the ride I usually prefare Fairline and Princess in head seas, they run flatter while Sunseeker's usually tend to be dryer boat.
What is annoying in most Sunseeker is the fast lowest planning speed of an average of 18 knots. That means if it is really rough you really have to go driving in displacement.

I have run a 62' 2007 predator immensely and over 5,000 NM's, which shares the same hull as the Manhattans. It is very dry, but it also is an excellent riding boat in a head sea. It is the best running boat in it's class/size that I have run. I've also run a 2014 56' Princess 800NM's and a 58' 2007 Princess express 1200 NM's, neither were a very good head sea boat at all and pounded. I've also run a 2004 64' Princess about 2000 NM's and it was better in a head sea than the other Princesses, but not as good of a ride as the Sunseeker. I've also run a 60' Fairline but it was a late 90's boat and ran well, but the interior didn't fare very well. The Sunseeker will stay on plane down to about 15.5 knots with the trim tabs down. The engine room on the Princess will kill you to work on anything, it's so tight. You have to rip out the salon carpet, pull the salon deck plates, just to do an annual servicing on that boat. The fairlanes are a bit better, but NOTHING like the spacious engine room in the Sunseeker.