Luffarrock,  The Dell Customized 5.1 U3 ISO you are referring to will work to update the host. The only differences in the Stock U3 ISO, and the Dell Customized version, is the hardware drivers present in the ISO's. Our ISO has drivers specific to our hardware, that aren't present on the Stock one. Beside that there's really no difference between them. I would recommend the Dell ISO, for the fact that upgrading to the Stock ISO can replace functional drivers with those that are not. You can also find information on the changes in U3 in the release notes here as well.   Let me know if this helps. 
Hello,   I am about to upgrade our VMware cluster. I want to upgrade it to ESXi 5.1 Upgrade 3 but am unsure if the stock ISO will run fine on a Poweredge M710 server?  I have had a look at Dells site and can find nothing that will work in Update Manager, only a custom ISO for re-installing.  Anyone out there successfully running a M710 on a stock U3 ISO? 
ESXi 5.1 U3 and M710