Per our FAQ, we technically don't allow most opinion-based questions. I'll instead focus on things to consider if you're choosing between a basic head unit/GPS bike computer (many of which aren't made by Garmin!) and a smart watch.
Some smart watches are all-rounders (the Apple Watches are one obvious example) and some are more sport-focused. We are all assuming that you’re interested more in a sport-focused wearable (my term, not sure if it’s a generally accepted one). The sport-focused wearables are the ones with the long battery life, although others have stated that GPS or other satellite navigation systems and the optical HRM will drain the battery life a lot faster than wearing the watch on standby. To some extent, this will depend on the unit’s power saving algorithms. Some wearables may have lower refresh rates than others, or the option to refresh at a lower rate.
This has been stated already, but to phrase it differently: a wearable is fine for logging your ride afterwards, but if you have it on your wrist, it won’t be as good as a bike computer. You may be fine with this. There are bike mounts for wearables, but to state the obvious, you will not be able to use the optical HRM if you mount a wearable on the bike.
You might reconsider how useful heart rate data are. As an estimate of calories burnt, heart rate is very imprecise. As a means of pacing interval training, heart rate is useful and a lot cheaper than power, but it wasn’t clear that you wanted to do this. As a side note, a lot of general purpose smartwatches can’t natively broadcast heart rate data to a bike computer. The Apple Watch is one of these. There may be software workarounds available, but these watches may not be designed to do this, and Apple may be particularly bad about this functionality. I believe that many Garmin wearables are able to broadcast HR data, but consider that Garmin is a sports and navigation company, whereas Apple is a computer manufacturer (and they like to tightly control their software).
If you’re involved in multiple activity types, that could skew your preference in favor of a wearable for the obvious reason that bike computers are specialist units.
One factor that nobody has addressed is your preference for gradient data. I am not sure how to check this, and I did look. In theory, any wearable with an activity profile for outdoor cycling might have gradient as a selectable field. However, I checked the manufacturer pages for a few major wearables, and I couldn’t see any specifics about the cycling profiles. This could be because relatively few serious cyclists use wearables as a primary computer. In theory, I suspect that any GPS-equipped computer could be programmed to use GPS functionality to estimate gradient, including a watch. I don’t know which ones have been programmed thusly. (It’s generally accepted that if you want accurate elevation and gradient data from your head unit, it should have a barometric altimeter, which not all do. I am not sure how prevalent this capability is among wearables, but I’d guess it’s not common, and especially among more entry level wearables.) As with heart rate, you could consider if accurate, real-time gradient info is a critical feature. If you upload your ride to Strava, you will be able to check gradient after the fact.