Which should I get? Is the domino a toy for a toy shop? It seems... Erm... a little limited, maybe. You have to use proprietary store bought dominos. You trade speed for flexibility. And you wouldn't get the aesthetic appeal of a true through tenon. But are those really non issues when the speed gain is factored in?
While I'm not interested in paying their prices, Festool tools seem to be really high quality. I would expect the domino tool to work quite well in a production environment.
Is it limited? Oh yeah. It's a one use tool: it cuts holes for loose tenons. Just like any one-use tool, there's not much else it's good for.
As to whether the trade-off is worth it... For me, personally, it's not. But then, I don't do much finish-quality work, and I'm more interested in the process than the result (which may explain why my results are often not very good. That might just be that I'm still learning, though). I'd rather take the time to cut real mortise and tenon joints, even if it takes five times as long.
But is the trade-off worth it for you? I have no way of knowing. It sounds like you want the speed of a powered cutter with the strength of a mortise and tenon. There are a lot of ways to do that, but the Domino is probably one of the fastest. If you cut your own loose tenons, you're not tied to their manufacturing, which would be my biggest concern, and it's not like a biscuit where it needs to be formed of some sort of compressed stuff that expands with glue.