The two overlap, they compete in the same auctions, and Google generally prioritises PMax when both are eligible. So "run both and see" is not a strategy — it is a way to make your data unreadable.
What you give up with PMax
- Search term visibility. You see far less of what you paid for.
- Placement control. Spend moves between Shopping, Display, YouTube and Gmail without you choosing.
- Brand separation. Unless you fence it, PMax absorbs brand traffic and reports it as performance.
What you get
Reach across surfaces you cannot easily buy separately, and automation that genuinely works once it has enough conversion data and a clean feed to work with.
When standard Shopping still wins
- Low conversion volume. PMax needs data; standard Shopping with manual control is more predictable while you build it.
- You need to see search terms — for a new store, that visibility is worth more than the reach.
- Catalogue problems. Fix the feed with something you can observe before handing it to automation.
If you run both
Separate them by product set, not by hope. Overlapping inventory means they bid against each other and you pay for the privilege.
More on fencing PMax in: the Performance Max page.