I subscribed to both over the years, including both services last year.
I typically used them as follows:
GC: 1) via Apple TV app
HS: 2) via computer browser on HDMI to TV
HS: 3) via iPad AirPlay to Apple TV
1) had the best picture when my internet was on and "fresh". It occasionally would drop bitrate in "adaptive" mode and would get grainy, but rarely, rarely would buffer. Top quality product. Biggest beef: some nights you don't get the commercials/intermissions, and being east coast that would make me fall asleep during intermissions
... sounds funny, but being in the US, I appreciated the HNIC, the Canadian commercials and the "immersiveness" of commercials tieing it all together
2) Perhaps not as crystal clear as GC, but at it's best, HS is a very close contender. The stream reliability has better this year than ever before. Full commercials and home or away feed are awesome. GC really has tough competition. Oddly though... I can't reliably airplay the computer/browser feed to my apple TV, so wired was required
3) Hockey streams iOS version was the most reliable for airplay... so if I went to bed (no computer hooked up) I'd use this method...
Oddly, I got so used to using 2 & 3 that I almost forgot that I paid for GC. So... both are good choices.
I think GC is moving in the right direction with reducing the number of blackouts, but it is still such a big turnoff (come playoff time especially, when NBC/CBC are showing the games... and in NBC's case, on NBCSN which is not available Over The Air.
These online streaming services should be *alternatives to a cable subscription (just like Hulu, HBO Now, Netflix are to TV), and if they were, there would be no legit/legal reason for HS to exist. We should all be able to choose which service we want to pay for to get the hockey that isn't available OTA. I wish they'd figure that out.