I found this comparison information between PieSync & Zapier at this link: or read below:
Happy reading Sumolings!

PieSync is the answer to the first 3 questions on Zapier's FAQ page:
1
Can you fully sync my data between two apps?
Zapier doesn't currently offer two-way syncing features like version merging or version conflict resolution and most supported apps don't have triggers for updated data.
Yes, in both directions. Conflicting data is merged in a controlled way – fields will never be overwritten.
2
If I update something in one app, will that change be reflected in the other app?
In most cases, no. (...)
Yes. PieSync constantly monitors the records in both web apps, and when it detects changes, it updates the data in the other app too, keeping everything in sync.
3
Do you work with existing data? (i.e. data that existed prior to making a Zap...)
Data that was created before your Zap was turned On will not be acted upon.
Yes. During the initial sync we match the existing, historical data between the two systems.
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Read more about different usecases for PieSync and Zapier.
vs
Synchronizing apps
PieSync focuses on bringing apps together by fully syncing data between apps, automagically letting you work with the same data in different apps. It is designed to synchronize contacts in both directions, in real time.
Pushing data to apps
Zapier is more about 'zapping' data from one app into another, automatically adding information to help you automate tedious tasks. It works with many apps but only offers one-directional triggers.
When to choose PieSync?
• If you want to work with the same, correct and updated information in different apps simultaneously. Like in teams, or working remotely and on mobile.
• When you need duplicate prevention, data merging and updates on both sides.
For 2-way real time sync, PieSync is the way to go.
When to choose Zapier?
• If you need an if-this-then-that scenario, like a message appearing in the company chat every time somebody signs up for your newsletter.
• When you don't need the apps to talk to each other in both directions.
For 1-way, one time actions, Zapier is great.