This isn't a cost decision
Both options are free, or close enough to it that price shouldn't be what decides this. Notion and Google Sheets can each track everything a trading journal needs: date, instrument, entry, stop, exit, P&L, R-Multiple. What actually differs is how each one handles the question underneath the raw log: which setup is working, and how do you see that without hunting for it.
Pick Notion if you want the structure to do the work
Notion's advantage is relational. Link a Trade Log to a Playbook database, and the Playbook can carry a rollup that automatically shows win rate and average R for each setup, updating as you log new trades. You don't build a pivot table or filter anything by hand. The cost is setup time upfront: getting the relations and rollups configured correctly takes longer than opening a blank spreadsheet, and if you're not already comfortable with Notion's database features, there's a real learning curve before it starts saving you time instead of costing it.
Pick Google Sheets if you want to see the math
A spreadsheet keeps every formula visible and editable in plain sight. Win rate, profit factor, and R-Multiple are each one formula away, and you can see exactly how each number was produced rather than trusting a rollup you configured once and haven't looked at since. It also works identically on any device without needing an account beyond a Google login, and it's the more portable choice if you ever want to move your data somewhere else, since exporting a spreadsheet is simpler than exporting a relational Notion database.
The honest answer
If you already use Notion daily for anything else, use Notion, the setup cost gets absorbed faster because you're already fluent in the interface. If you don't, or you'd rather understand every number by looking at the formula that produced it, start with Google Sheets. Either one, used consistently for three months, will teach you more about your own trading than switching between both while looking for the "right" one.
The template you'll actually open every day beats the one that looked more impressive in a comparison article.
Whichever you pick, the tool itself only gets you halfway. What you do with the data once it exists is a separate question, and it's the one that actually determines whether months of consistent logging changes anything about how you trade.