If you compare the checksums that Dell shows for both those files you can see that they match. This means that they are the exact same download/file.  The Dell customized ESXi 5.5 A01 ISO is the latest that I know of.
Hello,  As we are planning to deploy the customized version of ESXi 5.5, I would like to know if this is the latest one: [URL]   Thank you!
Dell Poweredge R620 ESXi 5.5 customized