It really depends, i am using a high end laptop and i will never go back to a desktop.
My trading is fully automated so i don't need a lot of screen to display all my charts, i can monitor my algorithm wherever i go, work on new projects wherever i am, ...
Even if i were trading manually and would need some screen size for my charts i still would go for a laptop over a desktop. Connect it to a ultrawide screen and you have all the screen size you need. Mobility is much more important for me so i can trade from home, the office, ... Another big advantage is the battery, no problem when power goes out. Just turn on the mobile hotspot on your smartphone and you can continue trading or doing whatever it is you where doing.
If you take care of your device, i use mine for nothing but managing my algo's and other trading/investing projects, so no garbage on my laptop at all, it can run forever without any problems. Cooling is important so i use an extra fan under my laptop as a stand and i never have any overheating problems at all.