Garmin is the better overall choice.
Smart features - The Withings is more of an ‘assisted smartwatch’ than a true smartwatch. You can’t see much of your data on the watch itself, and have to use the app for most things. Garmin shows almost all of your data right on the watch. You can customize it a lot through the app, plus it has NFC pay, message notifications, weather, alarms, and more. You can also change the watch face.
Health - Garmin has continuous heart rate monitoring, which seems to be more accurate than Withings, which checks it every 10 minutes. Withings does have an ECG, but unless you have a health issue, it might not be very useful. Garmin offers more fitness-focused metrics like Body Battery and Stress levels.
Fitness features - Garmin is far ahead. The Withings doesn’t have built-in GPS, and tracking heart rate during activities drains the battery quickly. Garmin’s Crossover has built-in GPS and more customizable tracking screens. It also has specialized metrics for activities like swimming, paddling, and running.
Training features - Withings doesn’t offer anything. Garmin includes full training tools, workout plans, and daily suggestions.
You can also turn off Bluetooth on the Garmin (not sure about Withings). It won’t make calls or messages, but you can still see notifications when it’s connected to your phone.
Battery life - Withings lasts up to 30 days, but adjusting heart rate frequency affects that. The Crossover lasts up to 28 days, and if you get the solar version, it can go up to 70 days. GPS activities drain battery a bit, but you can extend it by reducing GPS quality.
The Crossover looks way better in person than in pictures, so I suggest seeing it before deciding.