New XPS 13 versus old xps 13 7390 is it worth upgrading ?

There is more than one XPS 13 at this point (and in fact there were two different XPS 7390 systems when yours was available - one clamshell, and one 2-in-1).

Currently, there are both clamshell and 2-in-1 versions of the XPS 13 models, both of which use a CPU similar to what you have.  Yes, a new system will be faster - partly due to the CPU and partly due to the faster SSDs available.

There is also the XPS 13 Plus, which despite a similar name is a totally different system. Unlike the system you have or the XPS 13, this one uses a full-power CPU (yours and the standard XPS 13 use low-power CPUs).  The 13 Plus is an odd design choice.  The CPU uses far more power than the standard XPS 13, and the system isn't much larger, so it will run hotter and very likely throttle more, which defeats some of the purpose of the more capable CPU.   If you really need that much CPU power, an XPS 15 is a far better choice, as it can more readily cool the CPU.

If all you're looking for is more storage, and you have the clamshell XPS 7390, you can always simply upgrade the SSD.  However, if you have the 2-in-1 7390 (or purchase a new XPS 13 2-in-1), note that on these, the SSD is soldered in -- it's not upgradeable.  With the 2-in-1, a new system is your only option if you need more storage.