You can use either. Just remember to upgrade your vCenter and Update Manager to 5.5 before attempting to upgrade the hosts.  The vanilla ESXi 5.5 image from VMware and the Dell "branded" ESXi 5.5 image start of the same. However Dell adds in additional sensors and drivers.  I would recommend going with the Dell image.
Quick question: I am needing to upgrade my ESXi 5.1 hosts to 5.5.  I'm going to be using the VMWare Update Manager to perform the upgrade to my hosts which are R620 servers in an M1000 chasis.  Do I need to download the Dell specific VMWare ESXi 5.5 ESIi 5.5 .iso or can i use VMWare's 5.5 disk?  I've not done an upgrade since the origional install and we used the Dell disk at that time because it had the drivers but thought that with the update manager I would not need to use the Dell specific disk now.   Thanks.
M1000 and R 620 Blades/VMWare Update Manager