Are you sure the Samsung Galaxy S26 Ultra is safer than the Xiaomi 17 Ultra? Xiaomi doesn't display ads, and cryptocurrency wallets work perfectly fine on it. Any software wallet is unsafe, and for large cryptocurrency amounts, you should use a hardware cryptocurrency wallet in conjunction with it. If a user installs a lot of different software from external apps, they will have security issues on any Android phone.
I never had this s26 Ultra because it is a newer model of the Samsung S series, so I can't provide the most updated information about this unit. for me, the most secure S series in my experience is the S9 and S9 Plus, which do have a unique feature called the iris scanner. I actually use a two phones right now because I bought the second phone very cheap due to the LCD being slightly damaged but still usable and I only use it offline and I never connected it to the internet because I made it a cold storage.
The other one I use online; all transactions are unsigned. If I am accessing a site that I know is not safe to use on that phone, I always set it to safe mode so that no app can run if I accidentally click them online.
About your question, "Which one is safe?" I've had a lot of bad experiences with Xiaomi, including when they stopped supporting the Play Store and received a lot of popup ads, so the Samsung S26 Ultra is still safe for me, depending on the user. I know there are some people spreading right now that they had pop-ups or adware on their phones with this unit model but it still depends on the user, but you can easily get rid of them by switching to safe mode and removing those notifications or apps that you accidentally installed.
The safety of your wallet or anyone else's is still up to the user, so while I may only know how to protect myself from online attacks, my experience with the Samsung S series has been excellent in comparison to other models. The only issue is that when a unit is nearly end-of-support, it appears that they always bloat the unit, making it laggy.
I don't trust Xiaomi. I do program some Xiaomi phones when extracting their firmware; it seems that this brand has a built-in adware, and they always had.