I like Part II better in pretty much every way. The first game was in my top 5 of all time, but Part II might legit be my favorite game ever.
I suppose I can see where the pacing criticisms come from for Part II, although personally I thought the pacing was sublime, which really surprised me given the length and the fact that I thought Uncharted 4 had some pacing issues. A bit of Day 1 is the only time I felt like the pacing could improve.
We'll have to see how I feel on a replay though, but I feel like the first part of Summer in Part I (before meeting Henry and Sam), plus Boston before getting to Bill's town both dragged on more than any of Part II.
The story, I can see why people could prefer either one, but I liked Part II more. Part I is definitely a "tighter," more focused and straightforward story that executes incredibly well. But Part II is so much more ambitious in scope, complexity, and themes while still being fantastic in execution. I also really liked almost all the new characters (especially Lev!), and the world building was incredible.
Gameplay I don't really need to get into. Part II is just such a colossal improvement here.
So, yeah. I think it's indisputably Part II for me, but they really feel like two parts of a whole that are hard for me to separate in many ways. I can understand people going either way.