You already bought the $649 machine, right? You're just looking to see if you can commend yourself on a worthwhile purchase, or need to kick yourself in the butt for buying an the expensive one when you could have bought one for a quarter the price that does the same thing?
Well for making that decision, here's some fodder... The only thing fuzzy about that HF welder is the quality, and there's no invertor technology employed there. That HF welder is the big asss heavy lump of copper transformer that we talked about above. Works OK, but doesn't come with any of the adjustability that yours does. In fact, the only thing adjustable about that HF welder is weld pressure and the amount of time the power is applied (because you can let off the handle any time you want to).
So it kinda all depends on what you're looking for... That HF welder is old bulletproof technology, but the results will be only as good as the guy using it. Your machine is programmable so you can get dependable consistent results from one weld to another even if you have no idea what you're doing.
From what I've seen, in the small amount of poking around I've done for this thread, Telwin has a good reputation, and that spot welder seems to be well a respected portable unit. Not sure the same sentiment exists for the HF offering.