With a single Tiller subscription, you're free to use both Google Sheets and Excel if you also have a Microsoft 365 subscription. You can try both platforms using the same email address regardless of the authentication type you chose (e.g. you can still use Microsoft Excel if you signed up with a Google account and vice versa).
Google Sheets vs Microsoft Excel
Our Excel add-in only works for Excel installed or Excel Online via a Microsoft 365 subscription.
We offer the Tiller Foundation Template for both Google Sheets and Microsoft Excel. The functionally is basically the same between the two platforms and how the template works with some nuances that are just differences between Google Sheets and Microsoft Excel (e.g. how the Category column dropdown works on the Transactions sheet because of the differences in how Google decided to design data validation vs Microsoft's design).
The Tiller Money Feeds add-on for Google Sheets has the following features that the Microsoft Excel add-in does NOT have:
Manual account balance entry via the add-on sidebar
Refresh account connections via the add-on sidebar
Tiller Community Solutions add-on for installing community-built solutions that are compatible with the Foundation Template
We have happy customers using both platforms so it really comes down to preference and how important the features that are not available for Excel right now are to you.
You can always start fresh on either platform in the future and carefully copy/paste your data between the different spreadsheets if you decide you want to try one or the other at a later time.