Well, really isn't there that much difference.
-Frames are roughly the same and wouldn't be much different in strength and quality.
-Swing arms look pretty much the same.
-Levers and throttle look better and appear stronger on the dhz. +1 DHZ
-Exhaust is just a matter of style, but there both same effect wise.
-Both would need there bars replaced with pro tapers straight away.
-Now I maybe wrong but the Atomik looks to be crf70 plastics and DHZ crf50. Not that the plastics make an impact but the size of the bikes will be different.
-The Atomiks come with carlsbad tyre where the DHZ come with no name tyres. +1 Atomik
-They both have china USD non-adjustable forks.
-Front Brakes and rear brakes are both going to be pretty much the same, you'd need to bleed the brakes before you trust them. Possible DHZ would of done this before you get the bike... but thats only a slim MAYBE.
-The DHZ comes with a rear rim lock and the Atomik doesn't, no biggy it's a $20 item. +1 DHZ
-Both come with an adjustable rear shock but the DHZ comes with a GPX shock, so it comes with a fancy brand name... but realistically it is a better shock. +1 DHZ
-Atomik has a 160cc engine and Dhz has 140cc. +1 Atomik
- Carby on the Atomik is a 26mm Molkt and the DHZ a 30mm Mikuni.
So 2 point for Atomik 2 points DHZ. You could possibly get an Atomik for like 500-600 bucks and use the extra $$ to do some work and that way you get a more powerful engine and can buy the best suited shock for you, plus the extra little bits like rim lock that dhz had and possibly still pay less. I use the same carby these bikes come with, there easy to work with and tune for a newbie. They have pretty standard after sale support, I know the people that run the factory and it has changed in the last 12 months, the people are MUCH better.
Or get the DHZ and don't have to spend any more but run the chance of getting bored of the motor in a few months and the size of the bike is smaller so you might find it uncomfortable to ride. DHZ have great.. No I mean AMAZING after sales support. So if you have completely no idea about bikes you're better of paying extra for the help.
Possibly look in to a DHZ 160 which is also crf70 style like the Atomik... but realistically I think you're best getting the Atomik and modding it so it's exactly the bike you want and can feel like you have put your blood, sweat and tears... and time in to it!
Each to their own everyone may shut my opinion down but that what I reckon buddy and thats my reason! Enjoy