Is OWASP Zap better than PortSwigger Burp Suite Pro?

Yes OWASP ZAP is a good option as it's an open source so always preferred but Burp Suite Pro  will give you more options, its one of the best tool to have for pentesters so defo worth it.

First things first both are having their own merits, however in my personal experience ZAP can replace your burpsuite for sure considering the License. Also as the latest ZAP versions are covering more advanced techniques and spidering patterns with lots of options in it, it is worth considering ZAP. However remember that burpsuite from latest versions with inbuilt chromium and it's emerging plugin support (Installable jars) you can use burp to the fullest and you can keep it as a swiss knife for your web and app pentesting. Couple of extensions in burp pro are interesting especially the race condition one. I always prefer using Burp and at instances I go with ZAP.