As a runner, you might want to be able to see your CGM (Dexcom) numbers on your watch. If you choose the t:slim, you might NOT want to get their latest G4 pump, it would be hard to get info on your watch. I would suggest getting their pump that does NOT have CGM built in and instead use the Dexcom G5, that will use your iPhone as the receiver, the iPhone would then transmit to your watch. Downside is you would need to carry an iPhone when you run.
The Dexcom G4 pump would show all your data but you would have to pull the pump out to access CGM data