The Garmin range is pretty huge, so I've written an article which attempts to differentiate between the various models. It has been written for anyone wishing to choose a Garmin watch, not just for dedicated speed sailors.

medium.com/@mikeg888/which-garmin-for-watersports-e1000c43a868

If you are looking to buy a Garmin for GPSTC, you need to choose one which has been tested and approved. The list of GPSTC approved devices can be found at apps.garmin.com/apps/b8831776-4a07-4bb7-8f40-2c496ab655ba?tid=1