QEMU or VirtualBox?

Having used VirtualBox on both macOS and Windows for the last 10 years, I can say that, even if it were possible, I would never dream of using it or any other virtualisation software as a Windows 'portable' application. Of course, some VMs, once fully created on one PC, can be run 'portably' from an external source, providing the appropriate virtualisation application is installed on any other PC to be used (which also needs to have sufficient surplus CPU cores and RAM) but, in my opinion and experience, that is also only a practical option when a VM is installed on an external SSD (not a USB Pen-drive or HDD which are far to slow) connected via, as a minimum a USB3.0 interface. It is also necessary to bear in mind that the full installation of a 'modern' OS in a VM is likely to need around 40 GB of drive space dedicated for its use (minimum 64 GB for Windows 11).

By 'Portable VirtualBox' I presume you are referring to https://www.vbox.me which should really be considered abandonware. It has had no updates since VirtualBox 5.1.22, whereas the latest VirtualBox installation version available today is 6.1.36, and the VirtualBox development team is known to be currently working on Version 7. See https://www.virtualbox.org/wiki/Download_Old_Builds for how many versions of VirtualBox there have been released since 5.1.22 on 28 April 2017 was current. Anyone using this option should also consider that there is no support available should they have problems.

QEMU for Windows is still described on the site as follows "QEMU for Windows is experimental software and might contain even serious bugs, so use the binaries at your own risk." - https://qemu.weilnetz.de/w32/ and https://qemu.weilnetz.de/w64/ There is no claim on the site that it can be run portably on Windows and only standard installers are provided for 32-bit and 64-bit versions. Also, as QEMU is a command line only based application a separate third-party GUI application would be needed to be acceptable to many Windows users. I have found no claims anywhere else on the Web that QEMU can be run portably so, unless you have a reliable link to your source of your information, I'm sorry but I have to disregard it. However, I am aware, of course, that there is a separate Windows GUI app for QEMU which can be run portably - https://gitlab.com/qtemu/gui/-/tree/master, but that still needs an installed. version of QEMU and perhaps that has misled you. My own experience with QEMU has been largely restricted to Linux operating systems, where it's used in conjunction with KVM, which is built in to the Linux Kernel; and most users still seem to prefer adding a GUI interface, i.e. 'Virtual Machine Manager'.

those two things are quite different
quemu uses directly kernel functions while VB runs as a software and uses drivers to be installed to communicate with the host.
None of them is really portable, as also for quemu one needs often additional sw to make it useful. VB meeds installation of proprietary drivers on the host system, so I would not talk about portable here at all.