I used "Old" atlas roco track from the 80s-90s and handlaid my yard. I am not familiar with modern Atlas. However, there is a big diference between the Atlas flex I used and ME flex track. The ME appears to be non-tempered. It bends very easily, and it holds the bend, including any kinks your bending put in. I found it harder to get a smooth curve with it. On the other hand the Atlas appears to be a tempered material and is "springy". If you bend a piece, it wants to spring back to straight. When you bend it to a curve, it self smooths itself to the curve as you tack it down. So it is easier to make smooth curves with the atlas - - but (there always is a but), the tempered rail is very hard to keep curved at the joints. It is easy to get a kink at the joint. On the other hand, the softer ME is very easy to get a smooth joint. One way to work atlas is to solder the rail joiners at the joints on curvers before tacking it down but after you cut it to length, the inner rail has to be shortened.
So both types/brands have their advantages and disadvantages. i think handlaid is least expensive, and most flexible. Flextrack is next in flexibility and obviously more expensive. But you can do any radius curve you like. I think sectional track provides the least flexibility and costs the most. However, sectional probably goes down the fastest provided you don't have to cut and fit anything.