I may not be the best person to ask. 
I started training by power with a Polar 450 then moved up to a Garmin 1030.
Never got into Garmin’s platform much. I only cycle so not interested in all the other tracking options and metrics on Garmin. I actually liked Polar’s platform better.
Now I am strictly Xert for everything and happy with EBC on my Palm phone or Kingkong Mini (neither has a SIM card). EBC does everything I need. I never used half the stuff on the 1030 anyway.
I think I will opt for Hammerhead’s trade-in regardless. Not going to get $160 for the 1030 from anyone else as it has a bit of the blue halo problem. It has been a paperweight.
I doubt I’ll use all the stuff on the Karoo 2 either and will primarily run EBC.
Your decision really depends on what usage scenarios you see yourself benefiting from. For example, maps with TBT directions, climb profiling, segments, etc…
The Connect IQ platform is a dead-end IMO. There is only so much you can do with it and Xert is maxed out there. Great stuff for what it does but limited by the Garmin environment.
Android on the other hand is why Karoo has started to take off. Constant updates and new features are catching up to what Garmin does plus superior screen and touch control.
So for the best of both worlds (multi-featured head unit with option to load EBC natively) I would have to say Hammerhead has the edge.
For my purposes EBC on a $99 Android device (KKM) with no SIM does the trick most days. The downside is the KKM is not waterproof or ruggedized while the Karoo looks like you could use it a a hockey puck. I don’t intend to test that scenario.
The Karoo will just be another option for me to grab (surely one of these units must be charged enough for today’s ride 
). I might try TBT on the Karoo when traveling but honestly I usually pull my phone out to look at a map in more details including POI. Or flip to another app like Komoot or RWGPS on whatever Android device I’m using.
If Xert were to add GPX support for EBC that is all I would ever really want. Don’t need TBT. Just zoom in/out. YMMV