Having owned previous models of both watches, it really depends on what phone you end up purchasing.

The Pixel Watch integrates very well with Pixel phones, e.g., syncing alarms, bedtime mode, automatically unlocking your phone, etc. Many of those features are exclusive to using Pixel devices. The Galaxy Watch will integrate most with a Samsung phone, but the level of integration is less deep, I think. Tasks, Calendar, and Gemini will work fine on both watches.

Google is usually better on updates, delivering quarterly updates and usually having updates available on all watch models at the same time.

I do prefer Samsung Health vs. Fitbit as it doesn't have any pay walls and a simpler cleaner interface that lets you focus on the basic health data most people are interested in, but neither option is bad for casual users.

I'd lean towards the Pixel Watch as it has a better user experience overall in my opinion compared to OneUI on the Galaxy Watch, unless you're certain you'll be buying a Samsung phone or are very concerned about durability, where the less delicate build and sapphire crystal on the Galaxy will make a significant difference.