It would depend on how do you use your PC or what is its purpose. For me, the reason of buying such should determine if you will be purchasing a new PC or assembling it.
If you want to absolutely fit the your demands and needs, the best option will be assembling it - bringing in the parts that can fulfill your desired computer specifications. Like you want a gaming pc with lots of varieties, then you can make your computer hybrid.
But if your reason of buying such is not that too intense or not on a matter that could literally affect your performance/work done, then buying a new PC will be much convenient, more reliable and less hassle.