That’s really a judgement call and my recommendation is that you decide what is most important to you:

• Morality of the company
• Repairability
• Security
• Hardware I am not putting privacy here since you can achieve it with both brands (fairphones are compatible with e/os and other LineageOS derivatives, which are privacy focused ROMs).

If your main concern is either having the best hardware or the best security, go with pixel. If it’s repairability or having an ethically produced phone, go with fairphone. I am a user of GrapheneOS on a pixel myself, but am thinking of going for a fairphone whenever this pixel dies. I didn’t particularly like the fairphone 5 due to its size and design, but I really like the design of the FP6, so maybe I’ll buy the FP7 when it comes out ;)