Not saying it's the one to get... but between the Stingray and Nemesis, I'd get the Stingray. Slightly shorter. Floating center pin. Magnet switch.
The thing with mechs is that it's mainly personal preferences and nitpicking that separate one from another. You won't really know what design features you prefer until you've played around with at least one. By the looks of them, they're just "a metal tube with a button". Once you've been using some all day, every day, you may develop a preference for certain things. Like how the button works. Center pin design. Locking mechanism, etc. That's when you know what to look for. Until then, your best bet is to go for one that looks alright and isn't considered utter garbage.
This.^^^
For a first mechanical, especially if you're a beginner and looking to try out things, it's hard to not go wrong with a few standard cheapo-o clones that are popular right now.
Nemesis, Atomo, and Stingray are well regarded for the most part and will get you vaping. They're popular for a reason. Minus the occasional dud, for the average price of below $40, they are a steal, and certainly won't break the bank to try any given model out. And as mentioned before, try to search out and purchase the Hcigar version of any given model if possible, as they are generally considered the most consistent in quality control in the world of cheap mass produced chinese clones.
While I pretty much only use a Reo everyday, and it's the most perfect device for me, best keep that suggestion tucked away until you get experimentation and "shiny-itus" out of your system, especially if you're unsure of what you want at this point and are on a budget. You may turn out to hate un-regulated mechs. You may turn into a hardcore collector, and suddenly desire a shelf full of 20 mechanicals in your living room. Now, if you happen to develop a passion for RDAs with your new mechs, and want that to magically become a hassle free, all day, take out with you anywhere vape; I'll see you in the Reo forum a few months from now.
One day at a time...
Cheers