Difference between the "New XPS 13" and the "XPS 13 7390"

The 7390 is the latest generation. You might also find the previous 9380 still for sale. In the US, there are options in the XPS store for 13” and New 13”. The former is the 9380 and the latter is the 7390. The reason the numbering changed so much is that the XPS models used to be the 9000 Series to place them numerically above all of the Inspiron models, but Dell decided to put everything into 3000/5000/7000 numbering recently, so that was applied to the XPS. At least for now, the XPS line only has 7000 Series models.

Also note that the XPS 13 and 2-in-1 are now both called the 7390, whereas under previous convention the 2-in-1 would have been the 7395. Lastly, while both systems get Core 10th Gen processors, only the 2-in-1 model gets the ”Ice Lake” Core 10th Gen processors that use Intel’s new 10nm process and their new Gen 11 GPU. Those processors are a major architectural change and a significant improvement over their predecessors in terms of CPU and GPU performance as well as battery life, whereas the 10th Gen CPUs in the regular XPS 13 are yet another small iteration of the 14nm architecture that Intel hasn’t changed much since it was introduced with Core 5th Gen.

I can see your confusion since there are XPS 7390 2-in-1s and 7390 laptops.  

I would suggest you pick the system you want by ignoring the price, at least initially.  Then look through and see if you want a touchscreen or a 4K display.  Other things like drive size and memory differences are fairly normal and easy to decide.  Although, with Win 10, having a smaller drive may be a problem further down the line.

Knowing exactly what you want to use if for will also help guide you and once you have decided on components, then you can go through the cost to see what you will accept.  

I am still confused. It is said that the new xps 13 2 in 1 use the "ice lake", the newest version of processor. But with with the same i5, the new xps 13 can be upto 4.2GHz, whereas the xps 13 2 in 1 only up to 3.6.GHz