Hi,
You can directly install the DELL ISO images. The difference between the 2 is just that DELL ESXi customized ISO has Dell drivers installed on the ISO. Whereas the Vmware ISO doesn't.
I would suggest to upgrade using the Dell customized ISO and then use the VMware Update Manager to patch the host to the latest.
This way you know for sure you aren't missing certain drivers.
Regards,
Siva Pb