My opinion is if you spend 500 bucks on a bike, it will last you a few rides before something serious breaks and it'll be off the road so to speak. If you spend 750 or so, you'll get something that will last a lot longer and nothing major should break. Having said that, you'll still need to maintain them like ALL dirt bikes, be it a Honda dual sport or a budget chinese bike. they ALL need maintenance. the chinese might need a touch more.
The bad things I've heard about Atomik is the engine. Ducar are the budget of budget engines, and very poor. You might be able to make them last, but reputation says it's very poor and probably not. As for the chassis etc, I haven't heard too many bad things about it. So if you got an Atomik and changed the engine over to somethig else, this might be a good way to go. But more than 600 bucks.
Pitpro's have generally had good reviews, and my experiences have been good too. they have the Lifan engine, which is the middle of the range budget engine. Look after it, and you won't have any problems. The only real problems I've heard for these bikes is when people don't take care of them. Not oiling air filters, bringing them back with worn out engines after a couple of weeks, parts falling off because they never bothered to tighten stuff. That sort of thing. Best budget bike here is the Pitpro 125RR for 750 bucks. You could go a smaller one for 600, but if you're a bigger guy, it really is going to be very small. I have the 110 with 10" rims and the 125RR with 14"/12". Bigger really does make a difference.
My opinion, save up a bit more and your choice will suddenly become a lot bigger and you'll also have the option for much better bikes. I'd say 700-800 should get you something that you will be very happy with.