Different products that have different pluses and minuses, just like the other competitors. It is nice to have A/V devices in the system, so that’s a plus for harmony. But since the smart thing/harmony integration does work both ways now, that sort of evens out. You’re right of course that long-term harmony might not decide to continue that relationship, but those customers are their customers too. I don’t see them just dropping it anytime soon.
For me, the two big things that smartthings has right now are the echo integration, which is not available with harmony alone at the present time, and the ability to include locks in remote access rules. Harmony only allows control of locks from the local system, meaning when you are at home. That has some security pluses to it although since the account is a cloud account anyway they’re still vulnerable to hacking. But it does mean if you’re at the office, and you want to unlock your Zwave lock for your dog walker, there’s no way to do it with harmony. It’s easy with Smartthings.
The other factor, of course, is that the harmony extender is so new. I would want to wait at least a month to see what the reviews look like before making a decision. I also want to see what kind of a rules engine if any, harmony provides, since that’s a big weakness in the Smartthings system compared to a number of other competitors.
If harmony allows me to unlock a Z wave doorlock with a zigbee motion sensor inside the house, and An Ibeacon outside the house, has a decent rules engine, runs a lot of stuff locally, including a tablet app connection, I’d definitely consider switching.
But not without echo integration, that’s my make or break feature right now. 
Currently, Harmony is not compatible with Amazon Echo. However, I have forwarded your feedback as a feature request to the Product Management to be considered for the future developments. We cannot guarantee that this feature will be added but if it will be, an official announcement will be made. Thank you for sharing your feedback.
Best Regards,
PKHarmony