Both ideas would work fine with difference in downtime.
As per me there is no concerns if you have same LUN attached to old ESXi and New ESXi.
You can follow below option.
1.Attach same LUN to new ESXi as well old ESXi(where old ESXi has placed VM folders), deploy new vCenter and add both new and old ESXi to new vCenter , then straightway vMotion & storage vMotion your VM files to New storage and New ESXi ...There will not be any downtime. You can even add old and new esxi to the same new vcenter cluster and use DRS/SDRS. There are no any major changes in DRS & SDRS features from 5.0 on-words.
Note: You need to make sure vMotion/SVMOtion requirements are satisfied.