The appearance is more than aesthetic, the draw of atomizers will be different on both. The flat-top results in a tighter draw than the slant top, slope top, round top. Particularly with the Protank which draws through the bottom of the 510 connector. The flat-top presents a nicer fix to 22mm and 23mm atomizers. The slope top is a perfect fit for 18.5 mm atomizers like the Protank, as is the Provari which it emulates. But the fit does have an impact on draw in some instances. To me the zMax slope represents a good compromise for the airy Protank draw. And I feel a more consistent vape experience overall. Flat-top mods really reduce it's potentials.
You can augment or correct for the draw in three ways on the Protank: By very slightly opening the base of the atomizer from a hand-tight, an eighth turn or less to allow some air in. This will provide needed airflow to an inefficient coil or a slightly flooded Protank, if you draw like me at times. An eighth turn and a draw without firing will introduce some air to correct vacuum issues in this atomizer. It works. You'll see a small bubble or two if succesful. You will immediately get the atomizer getting expected vapor production. Also, by loosening it very slightly at the 510 connection from the batt. Not too much or you reduce the electrical efficiency at the contact point. It works on the flat-top, not meaninful though on the slope. Some nice things to love about the Protank when working right.
I don't understand why vendors don't do a better job of better describing the diameters of these mods and how they fit up. Post a pictured for cryin' out load. Very frustrating.
If you have some eGo atomizers they'll both work natively with the flat or round. Both have drip wells but I can't imagine wasting a VW mod for that.
A simple charger is all you need. They're cheap enough. Buy the body and save the markup. Make sure that two caps are provided! Ask or email. Get a box. They should come with a short and longer battery caps. The latter will better accommodate button top batt's. The boxed item should include it. These mods come with good brass springs. It makes a difference.
I like a very smooth controlled vape most times, on the cool side when chaining. But when I hit hard, warm and fast. This mod provides a robust response in both directions. Not as precise as a Provari or eVic but quite effective. Like I said earlier.
The mechanicals are fun and I've been through quite a few. Just ordered my first original Grand Vapor. But nothing meets variable wattage for sheer enjoyment and precise pedal to the metal power when you want it.
Good luck and enjoy!
You kidding? Too satisfying once you do. Oh and btw, I do acknowledge the advice from Tex and some of the others...but really $50 for a reliable VV/VW device is awesome. It's an outstanding start to something that may change your life. It has mine. Box mods are down the road for me. I'm still on the amusement ride buying the cheap tickets. 
