My partner and I have tried a few couple apps over the years.
It never really worked out. Every time, we ended up with our memories scattered across random apps that we eventually abandoned. Usually it was because the app got bloated with weird features, the price suddenly doubled, or we just got tired of logging in.
What bothered me most was realizing that all our notes and photos were sitting on some stranger's server, tied to an account we had to pay a fee to keep. If you stop paying, or if the company goes out of business, your own memories are essentially held hostage.
Last September, Between, one of the biggest couple apps, lost a lot of its users' photos and videos. A cleanup job meant for old, inactive accounts reached active ones too, and the company said the files couldn't be recovered. I read some of the reviews afterward. People had lost years of it: first dates, weddings, their kids as babies.
In Warmth, your entries stay locally on your phone and sync to your partner's device through a shared, private iCloud space. It's shared between the two of you, but private from everyone else (including me). Even if you never pay for the app, your memories are kept and they keep syncing between you two.