Try both and see which you prefer to use. I switch back and forth on occasion but primarily rely on EBC to run workouts.
One difference is Garmin will connect to sensors by ANT+ and EBC by BLE.
That may factor in if you want to ride a SMART workout as designed while you roam around in virtual world with only power and HR connected to the virtual app by BLE.
Lately I’ve been running Xert Magic Buckets (XMB) on my 830 while the workout runs on EBC so I can see how XMB performs while completing the workout.
If I’m having trouble completing a HIT workout on EBC I’ll change to Slope mode and keep trying.
If I continue to fail the workout intervals I’ll switch to XMB to perform ad hoc intervals. The result is I can fill the low/high/peak bucket targets for the day even though I didn’t perform the workout to spec. This also shows you how you can use XMB outdoors to meet your XSS goal for the day without any workout loaded on the Garmin. 
Here’s an example activity with Target Power enabled so you can see the original workout profile along with where I failed and switched to Slope mode, then made up the high/peak difference on XMB. I accumulated some extra Low pts due to my double warm-up and extended cooldown.
When I do this type of workout I save the results on EBC and discard the Garmin recording.