I went from a Nike + footpod, to a Garmin FR50 with footpod to the Garmin 405CX GPS watch.
The Garmin footpod was significantly more accurate than the Nike one, The 405CX is even more accurate than the Garmin footpod. TBH the main reason I bought it was to stop faffing about swapping the footpod between shoes, and also the extra features available on the 405 watch (intervals etc).
If your Suunto accessories are ANT+ compatible, then they should work with a Garmin watch. I've no experience of the Suunto products, but they appear to be strongly focussed on heart rate monitoring and how it affects fitness, where the garmins are more pace/distance focussed with added heart rate monitoring. So the choice depends on your approach to training.
DCRainmaker's web site is a mine of useful info, but hasn't reviewed the T6d. It does review the new Ambit model, so may have some insights into the Suunto range.