I’ve had both.
Had a TomTom Runner for about 6 months. Great watch, really accurate plotting (I mainly run on roads so its easy to see if it is wrong). Interfaced with the phone well to get runs uploaded quickly and easily to Strava etc. Battery life was impressive too.
But on Black Friday last year I saw a Garmin Vivoactive for £99 and snapped it up as it was nearly Christmas. Bought it because of the smart notifications stuff, customisation (different watch faces, workout readouts, “apps”, etc etc) and I knew I could ebay the TomTom for close to what I paid for it.
Equally good at tracking and doing the basic GPS watch stuff. The rest of the stuff it is clearly better than the TomTom at, but it is a higher tier product.
Wife has just got a garmin vivosmart plus – nice, accurate, does the fitbit thing (stairs, steps, sleep etc etc) but no cycling features.
So don’t be afraid of buying a TomTom. Garmin is the market leader, but TomTom knows GPS well too. It’s all about the features in the end, and your budget!